The genetic structure of eight summer and four winter colonies of the migratory bat (Tadarida brasiliensis mexicana) was examined to test the hypotheses that migratory populations are genetically distinct and that maternity colonies are composed of bats from genetically differentiated breeding colonies. Samples from seven of the summer colonies included only lactating females that were resident with their young in maternity roosts. Samples from winter colonies included adult males and adult females. Males and females were analyzed separately. Data from six polymorphic allozymic loci showed no statistically significant differences in allelic frequencies among colonies. Standardized genetic variances were low among all summer colonies (F̄st = 0.008) and higher among the winter colonies of males (F̄st = 0.016) and females (F̄st = 0.021). Cluster analysis based on genetic similarity provides no evidence that summer or winter populations are structured genetically into distinct geographic units. Analysis of genotypic distributions within colonies also provides no evidence that maternity colonies consist of bats from genetically differentiated breeding colonies. Data from 38 loci for a single summer colony showed that this colony contains a high level of genetic variability, as expected in an organism with a large, genetically effective population size.