The study of fitness costs associated with resistance to toxins from Bacillus thuringiensis Berliner (Bt) is important for understanding resistance evolution and for evaluating resistance management practices that prevent or mitigate resistance. A strain of European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) obtained from field collections throughout the U.S. Corn Belt in 1996, was selected in the laboratory for resistance to Cry1F by exposure to the toxin incorporated into artificial diet. The selected strain developed more than 3000-fold resistance to Cry1F after 35 generations of selection and readily consumed Cry1F expressing maize tissue. Using this resistant strain, a susceptible strain with similar genetic background and reciprocal crosses between them, we estimated fitness costs and their dominance by measuring fitness components and population parameters determined by fertility life tables. Comparison of life-history traits and population growth rates of genotypes homozygous and heterozygous for resistance relative to susceptible genotypes indicated existence of weak and recessive fitness costs associated with resistance. The significance of these results in relation to current resistance management practices is discussed.