Density and population structure of a native cecidomyiid gall midge,Rhopalomyia californicaFelt, and the spatial structure of its parasite guild have returned to normal at Woodside, Calif., several years after ecological disruption caused by repeated applications of malathion bait for eradication of Mediterranean fruit fly,Ceratitis capitata(Wiedemann). Midge larvae develop in terminal galls onBaccharis pilularisDC; galls are multichambered and can contain>50 larvae per gall (one larva per chamber). Samples taken during the summer–fall of 1989 and April 1990 (7–8 yr after the disruption) revealed that the number of larvae per 100 terminals, number of larvae per gall, and number of galls per 100 terminals were comparable at Woodside and the Jasper Ridge Biological Preserve, an adjacent nontreated area. The parasite guild consists of seven solitary species. Analysis of spatial structure of the guild in April 1990 revealed no significant differences (Woodside versus Jasper Ridge) for the following six parameters: parasitization by individual species, total parasitization, number of parasite species per gall (relative frequency), parasite species per gall as a function of gall size, total percent parasitization as a function of number of parasite species per gall (species dependence), and total percent parasitization as a function of gall size (spatial density dependence). The evidence obtained also illustrates how host population structure can influence parasite guild structure (i.e., direct relationship between number of host larvae per gall and number of parasite species per gall), and supports the view that there is a relationship between parasite guild structure and effect on the host population (i.e., direct relationship between number of parasite species per gall and total percent parasitization per gall). Some additional aspects of structure of parasite guilds are discussed.