The cecidomyiid midgeRhopalomyta californiaFelt is native to California where its larvae develop in terminal galls onBaccharis pilularisDC. Galls are usually multichambered and can contain>50 larvae per gall (one larva per chamber). Regression analyses involving>3,500 galls collected on 21 sample dates at two field sites in northern California revealed that average percentage of survival (larva to adult) ofR. californiawas either independent of, or directly related to, gall size (chambers per gall). No inverse density dependence was detected, suggesting that any intraspecific competition that may have occurred in larger galls did not reduce midge survival. This was confirmed in laboratory studies. The apparent inability of the midge's primary parasites to fully exploit host larvae in larger galls also may enhance midge survival in these galls. Under laboratory conditions, however, the mean number of eggs per female was significantly reduced in females that developed in larger galls, suggesting that intraspecific competition among larvae occurred. These patterns, if observed onB. halimifoliaL., could influence the effectiveness ofR. californicaas a biological-control agent for this weed in Australia and elsewhere. It is suggested that response to intraspecific competition be included in preintroductory evaluation of candidate agents in biological control of weeds.