The widespread planting of crops genetically modified to produce Bacillus thuringeinsis Berliner (Bt) toxins for pest control may affect nontarget arthropods. To address this issue, we compared the abundance and diversity of arthropods on plants in field plots of Bt cotton, non-Bt cotton, and a row mixture of 75% Bt cotton and 25% non-Bt cotton at two sites in Arizona. Over three sampling dates during 2 yr, we recorded all of the arthropods found on 120 cotton plants per treatment. This yielded 3,309 individual arthropods from 69 families. Excluding pink bollworm, Pectinophora gossypiella Saunders, the pest targeted by Bt cotton, we compared the abundance and diversity of all arthropods, chewing herbivores, sucking herbivores, rasping-sucking arthropods, and natural enemies. Arthropod abundance was significantly affected by site, plant height, and cotton type. More arthropods were collected from row mixture plots than Bt plots, but arthropod abundance did not differ significantly between Bt plots and non-Bt plots. The number of families collected was 57 for row mixture plots, 55 for non-Bt plots, and 47 for Bt plots. The number of families increased as arthropod abundance increased, suggesting that the differences in diversity among treatments were caused by differences in abundance. Within row mixture plots, arthropod abundance and diversity did not differ significantly between Bt plants and non-Bt plants. We conclude that the differences between Bt and non-Bt cultivars had relatively minor effects on the arthropod community on cotton plants.