The reduced fitness sometimes associated with resistant alleles has often been discussed as a possible tool of resistance management. A strain of Colorado potato beetles (Leptinotarsa decemlineata(Say)) from Massachusetts resistant to permethrin and azinphosmethyl produced 0.49 times as many larvae and 0.47 times as many adult females as a North Carolina susceptible strain. It also had a mean development time 3.5 d longer than the susceptible strain, resulting in a fitness value (intrinsic rate of increase) of 0.76 relative to the susceptible strain. An azinphosmethyl-resistant strain selected from the Massachusetts resistant strain but containing approximately 94% susceptible genome had a similar decrease in larvae and adult females (0.56 and 0.51 times, respectively) and an increased mean development time of 1.5 d. It was calculated to have a fitness value of 0.83 relative to the susceptible strain. However, azinphosmethyl-resistant heterozygotes (F1) showed no fitness disadvantage compared with the susceptible strain. Additionally, a permethrin-resistant strain also selected from the Massachusetts resistant strain and the permethrin-resistant heterozy-gotes (F1) showed no reduction in fitness relative to the susceptible strain. The implications of the reduced fitness due to the azinphosmethyl-resistance allele as a possible means of resistance management for Colorado potato beetle are discussed.