The maize cultivar Monumental MON810 was used to examine the environmental impact of the plant-expressed Bacillus thuringiensis toxin Cry 1 Ab. The Bt-maize was planted on five plots and alternated with five similar plots seeded with the non-transgenic parental cultivar. The whole field was treated with Guardian EC herbicide before sowing and again after maize germination. Samples of plants were analyzed for Cry 1 Ab expression and the presence of insects in 2 or 3-week intervals. The toxin was present in leaves and stems throughout the season and efficiently prevented maize attack by the European corn borer, Ostrinia nubialis, which occurred only on the non-Bt maize. Aphids, thrips and Orius bugs dominated among the plant-dwelling non-target insects. Their occurrence was not affected by toxin expression. The results indicate that the presence of Cry 1Ab in maize is lethal for O. nubialis, but innocuous for the non-lepidopteran insects that are associated with maize.