The impact of transgenic Bt-maize, expressing the Cry1Ab protein, on aphids, leafhoppers, cutworms and wireworms was evaluated at the farm scale by comparing their abundance on Bt-plots and those sown with the isogenic variety over three consecutive growing seasons. The impact of Bt-maize was different on each of the three-herbivore groups. There were significantly more aphids on the Bt-maize but in terms of aphid species, the difference was only statistically significant for Sitobion avenae and not for the other three most abundant species (Metopolophium dirhodum, Rhopalosiphum padi and Macrosiphum euphorbiae). The analysis of aphid age structure showed consistently more individuals on Bt-plots; differences were significant for alates, apterous adults and young nymphs of R. padi, apterous adults and apterous fourth instar nymphs of S. avenae, alates, apterous adults and apterous fourth instar nymphs of M. dirhodum. Leafhoppers (Zyginidia scutellaris), particularly mature nymphs, were also more abundant on the Bt-maize. In spite of this there was no difference in leafhopper damage to Bt and non Bt-maize. The reasons for this are unclear but may be due to changes in Bt-maize that favoured aphids and leafhoppers. Differences in aphid and leafhopper densities were not high enough to affect yield. However, they may have affected the availability of prey for polyphagous predators such as Orius sp. The Bt-maize did not affect the incidence of cutworms (Agrotis segetum) or wireworms (Agriotes lineatus). These results indicate that Bt-maize does not have a negative impact on the non-target maize biocenosis at the farm scale.