Incremental lines in the canine teeth of 20 stoats (Mustela erminea) corresponded exactly with known age in 5 males and 7 females marked as young, and with minimum-known age in 6 males and 2 females marked as adults. The lines are added in autumn (April and May) of each year, beginning when the animals are 6 months old. The correlation is accurate enough to permit determination of the ages of wild-caught adult stoats of unknown age. Two other techniques used previously (baculum weight, post-orbital ratio of the skull) are confirmed as accurate but much less precise methods of distinguishing young from adult stoats.