Eupelmus inyoensis (Girault) is a native, facultative secondary ectoparasite which has been reared from eight host species, representing six families and three orders of insects. This species is generally associated with the gall midge Rhopalomyia california (Felt) on Baccharis pilularis (DC) in California [USA], however, it is relatively rare and probably does not have a major impact on population dynamics of the midge. Analysis of galls containing E. inyoensis from the Jasper Ridge Biological Preserve and adjacent areas revealed that most contained only one individual of this species, regardless of the number of available hosts per gall. The evidence suggests that E. inyoensis has a relatively low reproductive capability; however, its broad host range and intrinsic competitive ability presumably enable it to persist in nature. Despite its rareness, this species appears well suited for coexistence in competitive parasite guilds.