The quality of an individual's odour can allow potential mates to discriminate against individuals of low social class, poor health status or unsuitable genotype. Competitive scent marking provides a further mechanism which could allow mates to discriminate between individuals of apparently high quality. The presence or absence of fresh countermarks from competitors within an owner's territory or area marked by a dominant animal provides a reliable indicator of the owner's ability to defend its territory or dominate competitors. This could be used by potential mates to discriminate between individuals advertising their apparently high competitive ability through their scent-marking behaviour and odour quality. We tested this by manipulating scent marks in the neighbouring territories of wild-caught male house mice, Mus domesticus. As predicted, oestrous females used scent marks to select males apparently able to defend exclusive territories over those unable to exclude intruders. Females were more strongly attracted to the odour of owners of exclusively marked territories and showed more sexually related behaviour when interacting with these males. Furthermore, while females preferred a territory containing a better protected nest site regardless of the owner's apparent competitive ability, they still used the presence or absence of intruder countermarks when selecting a potential mate. This suggests that females use scent marks as a reliable signal of the best-quality mate among neighbouring males independently of their nest location. Since assessment depends on both the territory holder's own marks and those of competitor males, countermarking is likely to be an important mechanism of competition for mates between neighbours. Copyright 1998 The Association for the Study of Animal Behaviour