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Multiple parasitoids, including multiple obligate special-ists, often coexist on a single host (for example Hawkins 1993b, Memmott et al. 1994). Explanations of parasitoid coexistence generally stipulate some difference between the competitors that allows them to exploit different re-sources, or a mechanism that allows for spatial or temporal differences in their attacks. In the case of specialist parasitoids with the same host, the differences are often subtle. There have been a number of articles discussing mecha-nisms or models developed to explain the continued coex-istence of such parasitoids, or similarly, of competitors for a single resource. A prevalent theme is that of a behavioral mechanism or physiological difference between compet-ing species, such as a trade-off between dispersal an