The food choice, food consumption, survival and development of cabbage looper, Trichoplusia ni, were determined on Bollgard II (Bt) and conventional non-Bt) cotton leaves in the laboratory. When larvae were offered a choice between Bt and non-Bt cotton leaves, 73.3% and 86.7% of larvae were found on non-Bt cotton leaves 1 and 8 h after inoculation, respectively. On non-Bt cotton leaves, all larvae initiated detectable feeding and consumed an average of 0.226 cm2 leaf area per larva in 48 h. In contrast, only 16.7% initiated detectable feeding with an average of 0.018 cm2 leaf area consumed by each larva in 48 h. When first instars were offered a mixture of a Bt and a non-Bt cotton leaf (a mixed test) with comparison with only non-Bt cotton leaves, their survival rates were not significantly different, but their development and pupal weights were significantly different. The pupae of both male (261.2 mg) and female (241.8 mg) developed from larvae in mixed test were significantly lighter than those of correspondent gender (210.8 and 198.9 mg respectively) in non-Bt cotton leaves. There were no larvae survived to adulthood on Bt-cotton leaves. The total developmental time of larvae in mixed-leaf test was significantly longer than that of larvae on conventional cotton leaves.