Galls ofRhopalomyia californicaFelt contain from 1 to>50 larvae per gall and can be viewed as discrete host patches in space and time. Analysis of>1,800 galls from three adjacent field sites in northern California revealed thatTorymus koebelei(Huber) parasitized hosts in a sufficient number of galls to permit an analysis of spatial density dependence for 10 sample dates. Average parasitization per exploited gall was consistently low (<25%), and response to spatial variation in host density was either density independent (five cases) or inversely density dependent (five cases). For all galls combined (exploited + nonexploited), average parasitization never exceeded 12%; response to spatial variation in host density was density independent in six cases and directly density dependent in four. Virtually all responses to spatial variation in host density (both data sets) qualified as “density vague.” Results indicate thatT. koebeleiis relatively inefficient in exploiting host patches and that it is not capable of consistently regulating or controlling the midge population. It is suggested that patch-exploitation patterns may be of some value in selecting parasites for use in biological control, particularly in augmentative release programs.