Female mating with multiple males in a single reproductive period, or polyandry, is a common phenomenon in animals. In this study we investigated variation in female mating behavior and its fitness consequences among three genetic strains of the red flour beetle, Tribolium castaneum. We found that the extent of polyandry and its fitness consequences varied significantly among the strains. In the first strain PRUZ, females mated multiply but incurred costs of polyandry in the form of reduced offspring production. Females of the second strain, NDG11, mated readily with multiple partners and benefited because polyandry led to higher offspring quality. Finally, TIW1 females were resistant to multiple mating and polyandry resulted in lower offspring production but improved offspring quality. Thus, in the first population we observed only costs of polyandry, in the second strain only benefits of polyandry whereas in the third we detected both costs and benefits of polyandry. Possible explanations for such a pattern are discussed. © 2007 Springer Science+Business Media, Inc.