The effects of host and parasitoid density on rates of parasitism byTrichogramma minutum, T. pretiosum, andT. exiguumwere tested in l.8-liter (13 cm diam by 13 cm high), 43-liter (23 by 36 by 52 cm), and 4,000-liter (2 by 2 by 1 m) arenas. Soybean foliage andHeliothis virescenseggs served as substrate and hosts, respectively. Rates of parasitism were significantly higher at the highest wasp density in all arenas. The effects of host density were not consistent among arenas. Rates of parasitism byT. minutumwere the highest in the l.8-liter and the lowest in the 4,000-liter arenas. Rates of parasitism byT. pretiosumexhibited the opposite trend. Rates of parasitism byT. exiguumdid not differ significantly from those ofT. minutumin the l.8-liter arena andT. pretiosumin the 4,000-liter arena. The cause of this lack of congruence in the relative performance ofT. minutumandT. pretiosumin the different sized arenas was not determined, however it may be a result of differential vertical stratification of searching activity by these two species.