An ecological assessment is made of the suitability of apples and kiwi fruit in New Zealand for transformation with genes from Bacillus thuringiensis (Bt). The assessment focuses on the risk of pests of these crops developing resistance to the Bt toxins expressed in the plants or in Bt used as a biopesticide. The key target pests which were assessed were tortricids. The most important ecological features of the crops and their pests which influenced the assessment were the host range of the pests, the availability of refugia of susceptible insects on non-transformed host plants within and around the crop, the mobility of the pests enabling the flow of genes for susceptibility into the selected pest populations, the ratio of susceptible to selected pest individuals, and insect behaviour in relation to Bt gene expression in the crop. Strategies are proposed for the management of transformed apple and kiwi fruit to maintain pest susceptibility to Bt, and if these are followed, the crops are considered suitable for transformation. It is essential that these Bt crops are deployed within an integrated pest management programme. The greatest risk of resistance to Bt is associated with polyphagous leaf rollers which also attack white clover, a crop which is a further candidate for transformation with Bt genes. This crop and its pest complex require full assessment of resistance risk before this step is taken.