Ectoparasite infestation has been implicated in increased physiological costs and reduced fitness of several animals. The semiaquatic habit of muskrat (Ondatra zibethicus) restricts its ectoparasites to small mites. We assessed variation in muskrat reproductive effort and fat content in response to relative abundance of parasitic mites. There were no significant correlations between total number of placental scars, number of litters, or percent body fat and the relative abundance of ectoparasitic mites. Our results suggest that these measures of muskrat reproduction and physiology are generally unaffected by arachnid ectoparasites.