Two data sets were compiled, the first representing the parasitoids known from 381 well-studied host species in 56 genera and the second representing the parasitoids from 389 populations of 66 host species, to examine between- and within-host-species variability in parasitoid complexes and to estimate levels of guild occupancy in parasitoid communities. Analyses were based on assigning parasitoids to each of nine guilds distinguished by the host stage attacked, mode of parasitism, and the host stage from which parasitoids emerge. Host taxonomy and host feeding type were used to evaluate the relative influence of historical and ecological processes on parasitoid guild structure. The number of parasitoid guilds that hosts support was highly variable both among host species and among populations within host species. Guild numbers were strongly associated with host taxonomy but weakly with host feeding type, suggesting that historical patterns of host and parasitoid evolution are much more important determinants ofparasitoid guild structure than are current biological and ecological characteristics of hosts. The bulk of the variability in the number of parasitoid guilds occurred among host orders and among host species within genera. Differences among host families within orders, and among genera within families, contributed relatively little variance. Summing levels of occupancy by parasitoid guilds within host genera indicated that a maximum of 69% of the potential parasitoid guilds that hosts could support are actually present. Using data from multiple populations within each host species indicated a maximum guild occupancy rate of 73%. Thus, we estimate that at least 25% of the parasitoid functional groups that could persist are absent from communities, suggesting that parasitoid communities are infrequently saturated with species. Across all parasitoid guilds, distributions of host utilization were not associated with either host taxonomy or host feeding type. However, levels of host utilization by specific guilds were associated with both variables. Our results are concordant with those arising from similar analyses of the guild structure of the insect herbivores of British trees, suggesting that undersaturation is widespread in insect communities at all trophic levels, and the functional structure of communities is largely due to chance historical events.