Studies of the effects of insect-resistant transgenic plants on beneficial insects have, to date, concd. mainly on either small-scale 'worst case scenario' lab. expts. or on field trials. We present a lab. method using large population cages that represent an intermediate exptl. scale, allowing the study of ecol. and behavioral interactions between transgenic plants, pests and their natural enemies under more controlled conditions than is possible in the field. Previous studies have also concd. on natural enemies of lepidopteran and coleopteran target pests. However, natural enemies of other pests, which are not controlled by the transgenic plants, are also potentially exposed to the transgene product when feeding on hosts. The redn. in the use of insecticides on transgenic crops could lead to increasing problems with such nontarget pests, normally controlled by sprays, esp. if there are any neg. effects of the transgenic plant on their natural enemies. This study tested two lines of insect-resistant transgenic oilseed rape (Brassica napus) for side-effects on the hymenopteran parasitoid Diaeretiella rapae and its aphid host, Myzus persicae. One transgenic line expressed the .delta.-endotoxin Cry1Ac from Bacillus thuringiensis (Bt) and a second expressed the proteinase inhibitor oryzacystatin I (OC-I) from rice. These transgenic plant lines were developed to provide resistance to lepidopteran and coleopteran pests, resp. No detrimental effects of the transgenic oilseed rape lines on the ability of the parasitoid to control aphid populations were obsd. Adult parasitoid emergence and sex ratio were also not consistently altered on the transgenic oilseed rape lines compared with the wild-type lines.