Studies were conducted to determine the effects of transgenic Bt-cotton on three larval instars of beet armyworm, Spodoptera exigua. First, third and fifth instars were fed field collected Bt-cotton leaves for 1, 2, 3, 4 and 7 d or until pupation, and then transferred to artificial diet. Larval mortality to pupation, larval survival to adults, larval and pupal periods and pupal weights were recorded. Larval mortality at pupation was low for all instars tested, and for the time period on Bt-cotton. Larval period increased with the increase of feeding time on Bt-cotton in 1^st and 3^rd instars, but not in 5^th instar, and significantly differed among three instars when fed continuously on Bt-cotton leaves. Pupal weight was reduced with increase in feeding time on Bt-cotton, and a greater reduction was recorded in larger instars when they were transferred to diet from Bt-cotton. Feeding time on Bt-cotton effected the survival to adults in 1^st and 3^rd instars but not in 5^th instars. These data indicate that Bt-cotton results in a low level of mortality in S. exigua larvae, however, it does adversely effect them in other ways such as reduction in pupal weight and extension in larval developmental time.