Abstract: Laboratory bioassays were conducted to assess the impact of pollen from thegenetically modified corn (Zea mays) event Mon810 on larvae of the Small Tortoiseshell (Aglaisurticae), a non-target butterfly species. Plants with the event Mon810 express the Cry1Ab proteinfrom Bacillus thuringiensis in all tissues and are resistant against the European corn borer. Thirdinstar larvae of A. urticae were fed with corn pollen deposited on leaves of its host plant (stingingnettle) in no-choice tests (I) once in a single dose experiment (1,000 Bt-pollen grains per cm2)and (II) over two and a half days to assess impacts of prolonged exposure. Additionally, a (III)single dose experiment with 9,000 pollen grains per cm2 was performed. Larval mortality,development, weight gain and feeding activity were observed. No statistically significantdifferences in mortality, developmental time and weight gain in comparison to control groupswere detected either in the single dose or the multiple dose experiment. However there was asignificantly lower feeding activity in the Bt-treatment groups in both the single and the multipledose experiments. Based on literature data on expected environmental concentrations of cornpollen a negligible risk for A. urticae larvae is determined in this study.