Females of a digger wasp Crabro cribrellifer nested in a 234 nest aggregation covering 2 x 65 m in area. Variability in 3 edaphic factors (soil temperature, hardness, and per cent moisture) had no significant influence on the non-random spatial distribution of nests within the aggregation. A significant positive correlation between wasp nest density and the mean number of parasitic flies, Metopia campestris, present was shown. Yet, based on 20 excavated nests, the probability of cell parasitism decreased as nest density increased. This may give females nesting within a dense aggregation a selective advantage, and supports the "selfish herd" model developed by Hamilton.