The serpentine soil-based grasslands of central California are severely threatened by non-native plant species and suburban sprawL At the present time only one animal species restricted to this community, the Bay checkerspot butterfly Euphydryas editha bayensis, is protected by the United States Endangered Species Act. This study examined the distribution of the butterfly and native grassland plant species across 27 habitat patches in the San Francisco Bay area in an effort to determine the value of the butterfly as an 'umbrella species'. Conservation activities designed for the Bay checkerspot butterfly were found to provide a tenuous protective umbrella for other elements of the grassland community. If all sites presently occupied by the butterfly were preserved intact, upwards of 98% of the native spring-flowering plant species would receive some measure of protection. However, if only the sites supporting the largest butterfly populations are preserved, or if portions of habitat patches classified as being of 'marginal' value to the butterfly are lost, then the proportion of plant species receiving protection drops substantially.