Resistance of the ship rat, Rattus rattus L. to warfarin

Abstract Wild-caught Rattus rattus from 38 populations from Merseyside and 3 other ports in England were tested for resistance. Animals were fed in the laboratory for 28 days on a bait containing 0·025% warfarin. The survival of 49·7% of 694 animals indicated exceptional resistance in 15 of the populations from Merseyside and 2 from London. Feeding patterns and delayed mortality provided additional evidence of resistance. The criterion of resistance and the significance of the findings are discussed.