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Crossing studies and enzyme electrophoresis were utilized to examine the genetic structure and relationships of populations of seven cismontane taxa of Astragalus sect. Leptocarpi subsect. Californici and five related species. The examined species were all self-compatible and no interspecific crosses were successful, demonstrating strong reproductive barriers. Forty populations were surveyed electrophoretically for 11 enzymes encoded by 17 putative loci. All conspecific populations exhibited high genetic identity values above 0.961. The mean genetic identity among all species was 0.69. Cluster analysis of genetic distance data indicates that the cismontane annuals (including A. gambelianus) are well-differentiated from the three desert species and the perennial outgroup A. nothoxys. Intraspecific genetic divergence and variation were low for both morphologically uniform and variable species, as well as for species with disjunct populations. Recent speciation is considered to be the most likely explanation for this pattern. Perhaps the ease with which reproductive isolation originates in the group has facilitated speciation and the ability of recently diverged species to become secondarily sympatric without genetic homogenization.