Laboratory selection increased resistance of pink bollworm (Pectinophora gossypiella) to the Bacillus thuringiensis toxin Cry1Ac. Three selections with Cry1Ac in artificial diet increased resistance from a low level to >100-fold relative to a susceptible strain. We used artificial diet bioassays to test F1 hybrid progeny from reciprocal crosses between resistant and susceptible strains. The similarity between F1 progeny from the two reciprocal crosses indicates autosomal inheritance of resistance. The dominance of resistance to Cry1Ac depended on the concentration. Resistance was codominant at a low concentration of Cry1Ac, partially recessive at an intermediate concentration, and completely recessive at a high concentration. Comparison of the artificial diet results with previously reported results from greenhouse bioassays shows that the high concentration of Cry1Ac in bolls of transgenic cotton is essential for achieving functionally recessive inheritance of resistance.