The larvae ofAsphondylia rudbeckiaeconspicuaOsten Sacken develop in ovoid chambers in polythalamous summer galls on the flower heads and apical meristems ofRudbeckia laciniataL. (Asteraceae). Three wasps are known to parasitize the gallmaker;Rileya americanaGirault,Torymussp. nearasteridis, andT. advenusO.S. In the current study we addressed three questions. Are gallmakers that occupy deep gall chambers less likely to be parasitized than those in shallow gall chambers? Does chamber depth vary with the number of chambers? And finally, does larval survival vary with the number of larvae per gall? At the three populations studied, unparasitized gallmakers occupied the deepest chambers, followed byR. americana, then byTorymusspp. Mean chamber depth increased with the number of chambers per gall at all sites, which raised the possibility that individual vulnerability to parasitoid attack could vary with the number of gallmates through the group effect on chamber depth. In one population, the rate of parasitism byTorymusspp. was highest in galls with an intermediate number of hosts. Thus, survivorship for the gallmaker at this site can be explained by the responses ofTorymusspp. to the number of chambers per gall. Patterns of parasitism forRileyawere marginally related to chamber number in another population.