Paired-choice assays in a tritrophic system have been carried out to study the influence of transgenic Bacillus thuringiensis var. kurstaki (Berliner) corn plants on prey preference of the predator Chrysoperla carnea (Stephens). Plants used were a transgenic B. thuringiensis-expressing (Cry1Ab) corn hybrid and the corresponding isogenic untransformed B. thuringiensis-free hybrid. Two different prey species were used in the experiments, Spodoptera littoralis (Lepidoptera: Noctuidae) and Rhopalosiphum padi (Homoptera: Aphidae). Both species were not lethally affected by the Cry1Ab toxin. C. carnea larvae were placed individually in a searching arena together with two groups of prey to choose from. One group had eaten transgenic B. thuringiensis corn (Bt+), the other non-transgenic corn (Bt?). Choice-experiments with various prey and host plant combinations were performed. The number and type of prey consumed by C. carnea, the time C. carnea larvae spent feeding on their chosen prey and the number of prey C. carnea only 'probed' upon for one minute or less were recorded. Observations were made for each of the three larval stages of C. carnea. When C. carnea could choose between S. littoralis fed transgenic corn (Bt+) and S. littoralis fed non-transgenic corn (Bt?), they showed a significant preference for S. littoralis fed non-transgenic corn (Bt?) as 3rd instars. Although not statistically significant, a similar trend was observed for the 2nd instar. No preference was observed when C. carnea had the choice between R. padi fed transgenic corn (Bt+) and R. padi fed non-transgenic corn (Bt?). This lack of preference for R. padi fed either transgenic or non-transgenic corn may be due to the absence of the Btprotein in the phloem. In prey combinations with S. littoralis and R. padi, all three larval stages of C. carnea showed a preference for R. padi regardless whether they had fed on transgenic or non-transgenic corn. These findings are discussed in context with biological control and pest resistance development.