The performance of Helicoverpa armigera (Hübner) on 15-wk-old cotton plants was compared for a susceptible strain, a near-isogenic laboratory-selected strain, and F₁ progeny of the two strains. Glasshouse experiments were conducted to test the three insect types on conventional plants and transgenic plants that produced the Bacillus thuringiensis (Bt) toxin Cry1Ac. At the time of testing (15 wk), the Cry1Ac concentration in cotton leaves was 75% lower than at 4 wk. On these plants, <10% of susceptible larvae reached the fifth instar, and none survived to pupation. In contrast, survival to adulthood on Cry1Ac cotton was 62% for resistant larvae and 39% for F₁ larvae. These results show that inheritance of resistance to 15-wk-old Cry1Ac cotton is partially dominant, in contrast to results previously obtained on 4-wk-old Cry1Ac cotton. Growth and survival of resistant insects were similar on Cry1Ac cotton and on non-Bt cotton, but F₁ insects developed more slowly on Cry1Ac cotton than on non-Bt cotton. Survival was lower and development was slower for resistant larvae than for susceptible and F₁ larvae on non-Bt cotton. These results show recessive fitness costs are associated with resistance to Cry1Ac.