Summary 1 The objective of this study was to examine the structure of parasitoid assemblages attacking soybean defoliator Lepidoptera in north-western Buenos Aires province, Argentina. 2 Through larvae sampling from 1992 to 1996, and laboratory data plus the known biology of the species recorded during the study, parasitoid species composition, richness, number of guilds per host species, and levels of parasitism, were determined. 3 Total species richness was 23, all species were primary endoparasitoids, and categorized as koinobionts. They belonged to Hymenoptera (11 species) and Diptera (12 species). Rachiplusia nu (Noctuidae) hosted the highest number of parasitoid species. 4 Four parasitoid guilds were recorded for R. nu (egg–prepupal endoparasitoid, early larval endoparasitoid, late larval endoparasitoid, and larval–pupal endoparasitoid); two for Spilosoma virginica (Arctiidae) (late larval endoparasitoid and larval–pupal endoparasitoid) and Colias lesbia (Pieridae) (early larval endoparasitoid and larval–pupal endoparasitoid); and one for Anticarsia gemmatalis (Noctuidae) and Loxostege biffidalis (Pyralidae) (early larval endoparasitoid). 5 Only four out of seven potential parasitoid guilds defined for Lepidoptera were recorded in a soybean agroecosystem located in north-western Buenos Aires province, suggesting that potential host niches were not totally utilized. 6 Soybean lepidopteran defoliators supported a mean species richness of 4.8 parasitoids, a result that is similar to that reported for exophytic hosts in the Neotropics. 7 Parasitoid assemblages from each host differed in species composition, richness, number of guilds, and levels of parasitism.