Laboratory experiments were conducted to compare relative fitness of strains of Colorado potato beetle resistant and susceptible to Bacillus thuringiensis subsp. tenebrionis Cry3A toxin. Net replacement rates and intrinsic rates of population increase were calculated for resistant and susceptible populations. During the experiment, susceptible males on average copulated 13.3 ± 1.5 times, while resistant males copulated only 8.0 ± 1.0 times. Susceptible females produced an average of 824.2 ± 68.1 eggs and 590.9 ± 58.5 larvae, which was significantly more than 484.6 ± 48.0 eggs and 334.9 ± 39.7 larvae produced by an average resistant female. As a result, both net replacement rate and intrinsic rate of increase were reduced for the resistant population. Furthermore, twice as many susceptible beetles as resistant beetles survived overwintering diapause. Our results clearly indicate that relative fitness of resistant individuals is reduced in the absence of Bacillus thuringiensis in the environment. Therefore, relaxation of selection pressure through refugia and insecticide rotation will favor decrease in the frequency of resistant alleles in beetle populations. [Full Text]