Carbaryl (1-napthol methylcarbamate) is being considered for control of fleas on prairie dogs (Cynomys spp.) used in black-footed ferret (Mustela nigripes) recovery in the western United States. The potential for relay toxicity in ferrets was determined by feeding carbaryl treated prairie dogs to black-footed ferret X Siberian polecat (M. eversmanni) hybrids. Adult prairie dogs were treated topically with 2.5 g of commercial 5% carbaryl dust sold as flea powder. After 14 days prairie dogs were killed and fed to ferrets. Potential for relay toxicity was evaluated by analyzing ferret blood cholinesterase (CHe), prairie dog brain Che, and hepatic carbamate concentration. There was no difference between pre- and post-exposure blood CHe activity, nor did treated prairie dog brain CHe differ significantly from controls. Post-exposure blood CHe did not exhibit reactivation after dilution in aqueous buffer. Hepatic carbaryl concentrations were less than detection limits (50 ppb). Based on these results, we conclude that short-term use of carbaryl for flea control on prairie dogs does not pose a hazard of relay toxicity in black-footed ferrets.