Galls of Rhopalomyia californica Felt 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 that Torymus koebelei parasitized hosts in a sufficient number of galls to permit an analysis of spatial density dependence. 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.