Many species of oak gallwasp (Hymenoptera: Cynipidae: Cynipini) induce galls containing more than one larva (multilocular galls) on their host plant. To date, it has remained unclear whether multilocular galls result solely from clustered oviposition by a single female, or include the aggregated offspring of several females (multiple founding). We have developed a novel maximum–likelihood approach for use with population genetic data that estimates the number and genotypes of parents contributing to offspring from each gall. We apply this method to allozyme data from multiple populations of four oak gallwasps whose asexual generations develop in multilocular galls (Andricus coriarius, A. lucidus, A. panteli and A. seckendorffi). We find strong evidence for multiple founding in all four species, and show the data to be compatible with multiple founding rather than founding by a single foundress mated with multiple males. The extent of multiple founding differs among species: in A. lucidus and A. seckendorffi most galls are induced by a single female, whereas in A. coriarius and A. panteli over half of the galls sampled were multiple founded. We suggest that variation in levels of multiple founding may be due to consistent ecological differences between the four species.