Rhopalomyia californica (Felt) is an endemic cecidomyiid midge which develops within terminal galls on Baccharis pilularis (DC) in California. The number of midge larvae per gall varied from 1 to 40 and individual galls were exploited by as many as five species of parasites. Total percentage of parasitization per gall was relatively constant. Instead of being density-dependent, total mortality was species-dependent-the proportion of hosts parasitized per gall was directly related to the number of species of parasites reared from the gall. The evidence is consistent with the hypothesis that this guild is structured so as to reduce the incidence of interspecific competition. The structure of a parasite guild may not necessarily be compatible with the goals of biological control. Parasite guilds such as this one might be restructured" so as to actually increase their efficiency in maintaining pest populations at relatively low levels. Additional aspects of restructuring parasite guilds are discussed. "