The effects of insect-resistant transgenic cotton on the diversity of arthropod communities in cotton fields in North China were investigated during 2000-2001. The field experiments were conducted in non-insecticide treated plots of transgenic cotton lines (SGK321, a transgenic cotton variety carrying Cry1A+CpTI genes; GK12, a transgenic cotton variety carrying the Cry1Ac gene) and their conventional parent varieties (Shiyuan 321 and Shimian 3, respectively), and in sprayed plots of the parent varieties where pyrethroid insecticides were used regularly against the cotton bollworm. All the arthropods were sucked up using a portable suction device and identified to species wherever possible. They were later sorted into guilds (pests, natural enemies and neutral arthropods) for diversity analysis using the method of Shannon's index. The diversity of arthropod communities in transgenic cotton plots was similar to that in conventional cotton fields without spraying, but Shannon's index for total arthropod community and the neutral arthropod guild in Bt cotton fields were significantly higher than those in sprayed plots in the mid and late growing stages of cotton. It is concluded that planting of Bt cotton could increase the stability of arthropod communities in cotton ecological systems and benefit the management of insect pests in cotton.