diff --git a/src/runtime/assemblymanager.cs b/src/runtime/assemblymanager.cs
index a791b8195..5d9759375 100644
--- a/src/runtime/assemblymanager.cs
+++ b/src/runtime/assemblymanager.cs
@@ -1,11 +1,11 @@
 using System;
-using System.IO;
 using System.Collections;
-using System.Collections.Specialized;
+using System.IO;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
-using System.Reflection.Emit;
+using System.Threading;
 
 namespace Python.Runtime
 {
@@ -15,12 +15,16 @@ namespace Python.Runtime
     /// </summary>
     internal class AssemblyManager
     {
-        static Dictionary<string, Dictionary<Assembly, string>> namespaces;
+        // modified from event handlers below, potentially triggered from different .NET threads
+        // therefore this should be a ConcurrentDictionary
+        static ConcurrentDictionary<string, ConcurrentDictionary<Assembly, string>> namespaces;
         //static Dictionary<string, Dictionary<string, string>> generics;
         static AssemblyLoadEventHandler lhandler;
         static ResolveEventHandler rhandler;
+        // updated only under GIL?
         static Dictionary<string, int> probed;
-        static List<Assembly> assemblies;
+        // modified from event handlers below, potentially triggered from different .NET threads
+        static AssemblyList assemblies;
         internal static List<string> pypath;
 
         private AssemblyManager()
@@ -36,10 +40,10 @@ private AssemblyManager()
         internal static void Initialize()
         {
             namespaces = new
-                Dictionary<string, Dictionary<Assembly, string>>(32);
+                ConcurrentDictionary<string, ConcurrentDictionary<Assembly, string>>();
             probed = new Dictionary<string, int>(32);
             //generics = new Dictionary<string, Dictionary<string, string>>();
-            assemblies = new List<Assembly>(16);
+            assemblies = new AssemblyList(16);
             pypath = new List<string>(16);
 
             AppDomain domain = AppDomain.CurrentDomain;
@@ -105,9 +109,8 @@ static void AssemblyLoadHandler(Object ob, AssemblyLoadEventArgs args)
         static Assembly ResolveHandler(Object ob, ResolveEventArgs args)
         {
             string name = args.Name.ToLower();
-            for (int i = 0; i < assemblies.Count; i++)
+            foreach (Assembly a in assemblies)
             {
-                Assembly a = (Assembly)assemblies[i];
                 string full = a.FullName.ToLower();
                 if (full.StartsWith(name))
                 {
@@ -266,9 +269,8 @@ public static Assembly LoadAssemblyFullPath(string name)
 
         public static Assembly FindLoadedAssembly(string name)
         {
-            for (int i = 0; i < assemblies.Count; i++)
+            foreach (Assembly a in assemblies)
             {
-                Assembly a = (Assembly)assemblies[i];
                 if (a.GetName().Name == name)
                 {
                     return a;
@@ -295,15 +297,15 @@ public static bool LoadImplicit(string name, bool warn = true)
             bool loaded = false;
             string s = "";
             Assembly lastAssembly = null;
-            HashSet<Assembly> assemblies = null;
+            HashSet<Assembly> assembliesSet = null;
             for (int i = 0; i < names.Length; i++)
             {
                 s = (i == 0) ? names[0] : s + "." + names[i];
                 if (!probed.ContainsKey(s))
                 {
-                    if (assemblies == null)
+                    if (assembliesSet == null)
                     {
-                        assemblies = new HashSet<Assembly>(AppDomain.CurrentDomain.GetAssemblies());
+                        assembliesSet = new HashSet<Assembly>(AppDomain.CurrentDomain.GetAssemblies());
                     }
                     Assembly a = FindLoadedAssembly(s);
                     if (a == null)
@@ -314,7 +316,7 @@ public static bool LoadImplicit(string name, bool warn = true)
                     {
                         a = LoadAssembly(s);
                     }
-                    if (a != null && !assemblies.Contains(a))
+                    if (a != null && !assembliesSet.Contains(a))
                     {
                         loaded = true;
                         lastAssembly = a;
@@ -362,16 +364,13 @@ internal static void ScanAssembly(Assembly assembly)
                     for (int n = 0; n < names.Length; n++)
                     {
                         s = (n == 0) ? names[0] : s + "." + names[n];
-                        if (!namespaces.ContainsKey(s))
-                        {
-                            namespaces.Add(s, new Dictionary<Assembly, string>());
-                        }
+                        namespaces.TryAdd(s, new ConcurrentDictionary<Assembly, string>());
                     }
                 }
 
-                if (ns != null && !namespaces[ns].ContainsKey(assembly))
+                if (ns != null)
                 {
-                    namespaces[ns].Add(assembly, String.Empty);
+                    namespaces[ns].TryAdd(assembly, String.Empty);
                 }
 
                 if (ns != null && t.IsGenericTypeDefinition)
@@ -383,14 +382,12 @@ internal static void ScanAssembly(Assembly assembly)
 
         public static AssemblyName[] ListAssemblies()
         {
-            AssemblyName[] names = new AssemblyName[assemblies.Count];
-            Assembly assembly;
-            for (int i = 0; i < assemblies.Count; i++)
+            List<AssemblyName> names = new List<AssemblyName>(assemblies.Count);
+            foreach (Assembly assembly in assemblies)
             {
-                assembly = assemblies[i];
-                names.SetValue(assembly.GetName(), i);
+                names.Add(assembly.GetName());
             }
-            return names;
+            return names.ToArray();
         }
 
         //===================================================================
@@ -471,9 +468,8 @@ public static List<string> GetNames(string nsname)
 
         public static Type LookupType(string qname)
         {
-            for (int i = 0; i < assemblies.Count; i++)
+            foreach (Assembly assembly in assemblies)
             {
-                Assembly assembly = (Assembly)assemblies[i];
                 Type type = assembly.GetType(qname);
                 if (type != null)
                 {
@@ -482,5 +478,92 @@ public static Type LookupType(string qname)
             }
             return null;
         }
+
+        /// <summary>
+        /// Wrapper around List<Assembly> for thread safe access
+        /// </summary>
+        private class AssemblyList : IEnumerable<Assembly>{
+            private readonly List<Assembly> _list;
+            private readonly ReaderWriterLockSlim _lock;
+
+            public AssemblyList(int capacity) {
+                _list = new List<Assembly>(capacity);
+                _lock = new ReaderWriterLockSlim();
+            }
+
+            public int Count
+            {
+                get
+                {
+                    _lock.EnterReadLock();
+                    try {
+                        return _list.Count;
+                    }
+                    finally {
+                        _lock.ExitReadLock();
+                    }
+                }
+            }
+
+            public void Add(Assembly assembly) {
+                _lock.EnterWriteLock();
+                try
+                {
+                    _list.Add(assembly);
+                }
+                finally
+                {
+                    _lock.ExitWriteLock();
+                }
+            }
+
+            public IEnumerator GetEnumerator()
+            {
+                return ((IEnumerable<Assembly>) this).GetEnumerator();
+            }
+
+            /// <summary>
+            /// Enumerator wrapping around <see cref="AssemblyList._list"/>'s enumerator.
+            /// Acquires and releases a read lock on <see cref="AssemblyList._lock"/> during enumeration
+            /// </summary>
+            private class Enumerator : IEnumerator<Assembly>
+            {
+                private readonly AssemblyList _assemblyList;
+
+                private readonly IEnumerator<Assembly> _listEnumerator;
+
+                public Enumerator(AssemblyList assemblyList)
+                {
+                    _assemblyList = assemblyList;
+                    _assemblyList._lock.EnterReadLock();
+                    _listEnumerator = _assemblyList._list.GetEnumerator();
+                }
+
+                public void Dispose()
+                {
+                    _listEnumerator.Dispose();
+                    _assemblyList._lock.ExitReadLock();
+                }
+
+                public bool MoveNext()
+                {
+                    return _listEnumerator.MoveNext();
+                }
+
+                public void Reset()
+                {
+                    _listEnumerator.Reset();
+                }
+
+                public Assembly Current { get { return _listEnumerator.Current; } }
+
+                object IEnumerator.Current { get { return Current; } }
+            }
+
+            IEnumerator<Assembly> IEnumerable<Assembly>.GetEnumerator()
+            {
+                return new Enumerator(this);
+            }
+        }
     }
 }
diff --git a/src/testing/Python.Test.csproj b/src/testing/Python.Test.csproj
index 56efda8e3..a0d62d4f9 100644
--- a/src/testing/Python.Test.csproj
+++ b/src/testing/Python.Test.csproj
@@ -122,6 +122,7 @@
     <Compile Include="indexertest.cs" />
     <Compile Include="interfacetest.cs" />
     <Compile Include="methodtest.cs" />
+    <Compile Include="moduletest.cs" />
     <Compile Include="propertytest.cs" />
     <Compile Include="threadtest.cs" />
     <Compile Include="doctest.cs" />
diff --git a/src/testing/moduletest.cs b/src/testing/moduletest.cs
new file mode 100644
index 000000000..8734f2569
--- /dev/null
+++ b/src/testing/moduletest.cs
@@ -0,0 +1,25 @@
+using System;
+using System.Threading;
+
+namespace Python.Test {
+    public class ModuleTest {
+        private static Thread _thread;
+
+        public static void RunThreads()
+        {
+            _thread = new Thread(() => {
+                var appdomain = AppDomain.CurrentDomain;
+                var assemblies = appdomain.GetAssemblies();
+                foreach (var assembly in assemblies) {
+                    assembly.GetTypes();
+                }
+            });
+            _thread.Start();
+        }
+
+        public static void JoinThreads()
+        {
+            _thread.Join();
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/tests/test_module.py b/src/tests/test_module.py
index f03954d28..a23f37d90 100644
--- a/src/tests/test_module.py
+++ b/src/tests/test_module.py
@@ -65,8 +65,11 @@ def testModuleInterface(self):
         import System
         self.assertEquals(type(System.__dict__), type({}))
         self.assertEquals(System.__name__, 'System')
-        # the filename can be any module from the System namespace (eg System.Data.dll or System.dll)
-        self.assertTrue(fnmatch(System.__file__, "*System*.dll"))
+        # the filename can be any module from the System namespace
+        # (eg System.Data.dll or System.dll, but also mscorlib.dll)
+        system_file = System.__file__
+        self.assertTrue(fnmatch(system_file, "*System*.dll") or fnmatch(system_file, "*mscorlib.dll"),
+                        "unexpected System.__file__: " + system_file)
         self.assertTrue(System.__doc__.startswith("Namespace containing types from the following assemblies:"))
         self.assertTrue(self.isCLRClass(System.String))
         self.assertTrue(self.isCLRClass(System.Int32))
@@ -353,6 +356,22 @@ def test_ClrAddReference(self):
         self.assertRaises(FileNotFoundException,
                           AddReference, "somethingtotallysilly")
 
+    def test_AssemblyLoadThreadSafety(self):
+        import time
+        from Python.Test import ModuleTest
+        # spin up .NET thread which loads assemblies and triggers AppDomain.AssemblyLoad event
+        ModuleTest.RunThreads()
+        time.sleep(1e-5)
+        for i in range(1, 100):
+            # call import clr, which in AssemblyManager.GetNames iterates through the loaded types
+            import clr
+            # import some .NET types
+            from System import DateTime
+            from System import Guid
+            from System.Collections.Generic import Dictionary
+            dict = Dictionary[Guid,DateTime]()
+        ModuleTest.JoinThreads()
+
 
 def test_suite():
     return unittest.makeSuite(ModuleTests)