We investigated the response of water voles to the odours of predators in outdoor enclosures. Water voles were given access to two food cages, one containing the odour of a predator and the other containing no odour, and were videotaped for 22 h. Sheep odour was also used as a control for novel odour. The predators were American mink, Mustela vison, and brown rat, Rattus norvegicus. There was no difference in the voles' response to sheep odour and no odour. The voles entered the treatment cages containing the mink's odour significantly fewer times than they entered the control cages, indicating avoidance of the former. They also gathered less food from, and stayed a shorter time inside, the treatment cages. The voles avoided the mink's odour more than the rat's odour. The fact that the experimental animals had not had previous contact with American mink suggests that the response is innate. These results also suggest that water voles perceived mink as a greater risk than rats, although they avoided odours of both these species. Copyright 1999 The Association for the Study of Animal Behaviour.