To examine genetic structure in populations of the mayfly Baetis tricaudatus and make inferences about its dispersal abilities, I analyzed 3 microsatellite regions in individuals collected from 14 locations in the San Bernardino Mountains of southern California. Genotypes separate into 2 strongly differentiated clusters, suggesting that B. tricaudatus as currently recognized consists of more than one species. Within each cluster, there is significant genetic differentiation among collection sites, but no significant trend of genetic isolation by distance. Heterozygote deficiencies exist at various sites for all loci. Gene flow appears to prevent isolation by distance across the 60-km stretch of the San Bernardino Mountains under consideration. Wahlund effects may account for the observed heterozygote deficiencies.