Abstract: Cotton leaf roller Sylepta derogata causes serious damage to cotton at the later growing stages of cotton. A leaf-dip method was used to measure the effects of low-concentration CrylAc toxic protein on the development and population growth of three successive generations of cotton leaf rol-lers in this study. The larval duration of Bt-fed S. derogata was 1.78-2.00 d longer, adult life span 1.44-1.94 d longer, and pupal mass 5.3%-11.8% higher than those of the control, respectively. The egg counts per female in the three generations were 65.1%, 47.3% and 37.6% higher, respectively, than those of the control, and the indexes of population trend were 11.4%, 17.5% and 20.0% greater, respectively. No significant differences were found in the duration of pupa duration, the emergence rate, the sex ratio, or the hatch rate. It was suggested that the population of cotton leaf rollers would steadily increase during the later stage of transgenic Bt cotton growth, therefore, further risk assessments of transgenic cotton and comprehensive management of these pests in cotton fields were needed.