This study was conducted to genetically map a potentially new wheat leaf rust resistance gene (LrW) using a novel genetic method and to test its effectiveness against current races of leaf rust (Puccinia triticina Eriks.) in Canada. Undoubled haploids of a near-isogenic line of Thatcher carrying the resistance gene (RL6107) were pollinated with a contrasting susceptible cultivar to generate an array of hybrids with random deficiencies arising from irregular meiosis of the haploid. Genetic analysis of the deficiencies in such populations can be used to locate qualitative traits by which the two parents differ through a process that we have called haploid deficiency mapping. In the present case, 5/417 hybrids were both susceptible to leaf rust (i.e. lacked the resistance gene) and also lacked several polymorphic microsatellite alleles from RL6107 that are specific to chromosome 5B. This correlated failed transmission of the resistance gene and deficiency for chromosome 5B. Analysis of an F2 population showed that the factor conditioning resistance was located on the short arm of 5B, 16.5 cM distal to the locus of the microsatellite Xgwm443. Since no other leaf rust resistance genes have been mapped to this region, LrW was re-designated Lr52. RL6107 was tested with 29 isolates of P. triticina, encompassing a diversity of virulence found in North America, with none showing virulence. The effectiveness and novelty of Lr52 make it a promising source of resistance for North American wheat cultivars.