Mesochorus curvulus Thomson was the only secondary parasite (hyperparasite) reared from six primary parasites (Braconidae: five Peristenus spp., one Leiophron sp.) collected in the field, in their mirid hosts. These six braconids parasitized nymphs of four species of phytophagous plant bugs, two (Leptopterna, Trigonotylus) feeding on forage grasses, and two (Adelphocoris, Lygus) feeding on alfalfa. The principal samples were collected weekly and biweekly on commercial farms in northwestern New Jersey over a 10-yr period. Although M. curvulus attacked six primary parasite species, it preferred Peristenus pallipes (Curtis), which in turn preferred the two non-native, grass-feeding mirids. These preferences suggest that both P. pallipes and M. curvulus are also not native to North America. Rates of hyperparasitism were higher in the most abundant primary parasites, so were density-dependent. The low hyperparasitism rates observed (1–11%) indicate that Mesochorus does not have a strong negative effect on the primary parasites, several of which are useful in biological control of plant pests. Diapause in Mesochorus appears to be regulated by the primary parasite rather than by photoperiod. The sex ratio of M. curvulus was normal (55% female).