none Tests of host discrimination by the external, gregarious parasitoid Oncophanes americanus Weed demonstrated that host size plays a significant role in determing both the number of eggs oviposited and the sex ratio of the parasitoid progeny. Number of larvae developing per host averaged 1.4, 3.1, and 8.2 for 3rd-, 4th-, and 5th-instar hosts, respectively. A high percentage of 2nd- and 3rd-instar hosts were stung without oviposition. The percentage of female adult parasitoids emerging from hosts increased from 62% to 3rd instars to 81% for 5th instars. Superparasitism was defined in terms of the efficiency of host resource conversion (ERC) by the parasitoid larval cohort. ERC is equal to the ratio of the biomass of the mature larval brood to the initial host biomass available for consumption. For O. americanus parasitizing larvae of the leafroller Argyrotaenia citrana (Fernald), significant increases in mortality of parasitoid larvae occurred at values of ERC gt 0.6.