The aggregation of parasites among individual hosts is one of the best documented features of parasite populations; we still do not know, however, why certain parasite species are more highly aggregated than other, related species. Here we search for a general explanation of interspecific variation in aggregation levels, based on the relationship between parasite body size and fecundity, transmission success, and intensity-dependent population regulation. We test the prediction that larger-bodied parasite species are more weakly aggregated than smaller-bodied related species, in a comparative analysis across parasitic nematode species. Across species, the variance-to-mean abundance ratio correlated negatively and significantly with nematode body sizes, as predicted. All other tests, however, including the more robust analyses controlling for phylogenetic influences, failed to support this result. This is mainly because the variance in infection levels is almost completely explained by mean parasite abundance. For this reason, it may prove difficult to identify a general biological explanation for interspecific variability in aggregation levels among parasites.