Abstract To assess the ecological effects of Bt -cotton cultivars (ZMSJ and ZMKCKC, expressing Cry1Ac/Cry2Ab and Cry1Ac/EPSPS, respectively), laboratory experiments were performed to evaluate the transmission of Cry1Ac protein in the food chain and the effects of Bt toxin on the ladybeetle predator Propylaea japonica through its herbivorous prey Aphis gossypii . Trace amounts of Cry1Ac protein (3.0 ng/g fresh mass [FM] in ZMSJ, 3.8 ng/g FM in ZMKCKC) were detected in A. gossypii feeding on Bt cotton cultivars. Cry1Ac protein was also detected in ladybeetles preying on Bt -fed aphids, and its quantity increased as the adult period extended (5–20 d). However, there were no distinct differences in the total developmental mortality, newly emerged adult weights, or fecundity between predators fed Bt -fed or Bt -free aphids. A delay in development was observed when P. japonica was fed A. gossypii reared on both Bt cottons. The preoviposition period was significantly longer in the ZMSJ ladybeetles than in the control ladybeetles. These results suggest that the Cry1Ac protein expressed in these transgenic cotton plants can be transmitted to predators through their herbivorous prey, but they do not affect their biology.