The Streptanthus morrisonii (Brassicaceae) complex is a group of six narrowly-distributed obligate serpentine endemic taxa whose habitat is threatened by geothermal development. Isozyme analysis of this little-studied complex supports the delineation of two species, S. morrisonii and S. brachiatus, but is at odds with the treatment of two subspecies based on morphology. These results may be influenced by small sample sizes but genetic studies of other Streptanthus taxa have shown patterns of relatedness that often transgress subspecies boundaries based on morphology. The present study further shows that members of the S. morrisonii complex share high genetic identity values (mean = 0.87) and are not genetically depauperate (mean value for percent of loci polymorphic = 37%, average number of alleles per locus = 1.48, and average heterozygosity per locus = 0.137). Preservation of their serpentine outcrop habitat is essential to the survival of these plants.