The effects on nontarget species of transgenic maize producing the Cry1Ab toxin of Bacillus thuringiensis (Bt) was evaluated in field trials. Ostrinia nubilalis larvae were collected in 1998 from transgenic Bt maize (Novaris Hybrid 176) and non-Bt maize at 4 geographical sites. At all sites, larvae from transgenic maize displayed a lower level of parasitism by the tachinids Lydella thompsoni and Pseudoperichaeta nigrolineata than did larvae collected from isogenic non-Bt maize. The difference was highly significant. In 1999, a field experiment at 2 sites compared the temporal abundance of nontarget arthropods in Bt maize (Monsanto Hybrid MON810) and isogenic non-Bt maize. The insects studied included Metopolophium dirhodum, Rhopalosiphum padi, Sitobion avenae, Orius insidiosus, Syrphus corollae [Eupeodes corollae], Coccinella septempunctata, Chrysoperla carnea, thrips and hymenopteran parasitoids. For all species but one, the number of individuals varied greatly over the season but did not differ between the types of maize. The only exception was thrips which, at one site, was significantly more abundant in Bt maize than in non-Bt maize. However, this difference may be an artifact.