Rhopalomyia californica Felt is a native cecidomyiid midge that develops in multichambered, terminal galls on Baccharis pilularis DC in California. At two study sites, number of midge larvae per gall varied from 1 to > 40. However, distribution of progeny for two of the midge's primary parasites, Torymus baccharidis (Huber) and Tetrastichus sp., was relatively constant, averaging between one and three individuals per gall, regardless of number of hosts per gall. It is suggested that species of parasites that are ineffective in their native homes may be of considerable value in classical biological control, especially when such ineffectiveness is due largely to interspecific interactions, rather than low reproductive capacity.