Field studies were conducted in 1992 and 1993 in Hermiston, Oregon, to evaluate nontarget impacts of transgenic Bt potato (Newleaf) expressing Cry3Aa protein derived from Bacillus thuringiensis (Bt) Berliner subsp. tenebrionis and conventional insecticides for control of the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say). Pitfall traps were used to estimate the abundance of nontarget ground-dwelling arthropods in different treatment plots. There were no significant differences in the trap captures of major ground-dwelling coleopteran predators such as carabids (Coleoptera: Carabidae) and staphylinids (Coleoptera: Staphylinidae) among Bt and non-Bt potato fields treated with weekly sprays of a microbial Bt-based formulation containing Cry3Aa, biweekly applications of permethrin, early and mid season in-furrow applications of systemic insecticides (phorate and disulfoton), or no insecticides. While weekly sprays of permethrin significantly reduced the trap capture of ground-dwelling spiders (Araneae), there were no significant differences in the capture of spiders between Bt and non-Bt-potato fields treated with Bt sprays, systemic insecticides, or no insecticides. Significantly more springtails (Collembola: Entomobryidae and Hypogastruridae) were captured in potato fields treated with permethrin than with any other CPB control regime (transgenic Bt potato alone, microbial Bt formulation containing Cry3Aa, applications of systemic insecticides, or no action controls), among which there were no significant differences. The relevance of these findings to field evaluation of nontarget impacts of transgenic Bt crops is discussed.