Brillouin-gain spectra are measured for pure-silica core, GeO(2)-doped core, and P(2)O(5)-doped core single-mode fibers with different index profiles. A narrow-linewidth semiconductor laser operating at 828 nm is used as the pump light source. The spectral shape of the Brillouin gain is found to be strongly related to the refractive-index profiles. The Brillouin linewidths evaluated experimentally are 90 and 215 MHz for a step-index single-mode fiber and a graded-index single-mode fiber, respectively. The Brillouin Stokes shifts depend on the core/clad-ding dopant materials and their concentrations and range from 20 GHz for the P(2)O(5)-doped core fiber to 21.6 GHz for the puresilica core fiber in the 0.8-microm-wavelength region.