There is very little information on the ecology of stoats Mustela erminea in the Iberian Peninsula, the southwestern limits of their Eurasian range. The aim of this study is to gain more knowledge of some ecological aspects of Iberian stoats, principally habitat and diet, by direct observation and scat collection in a valley in the Cantabrian Mountains, northwestern Spain. Stoats were observed in all the elevation range of the valley (820-1350 m a.s.l.) in different habitats including hedgerows-meadows, river and mountain meadows, normally close to woody vegetation. The body size of individuals killed on the road was larger than that reported for stoats in the Spanish Pyrenees, and winter whitening was complete. Stoats coexisted with common weasels Mustela nivalis, but in different habitats. According to scat analysis, stoats had a relatively varied diet of small mammals, insects, eggs/birds, fruit, reptiles and earthworms. In autumn-winter, small mammals and fruit were clearly the most abundant items in their diet, but in spring-summer it included more food categories. In the upper valley, most of the food items were fruits, and in the middle and lower valley, small mammals. Amongst the small mammals, mostly Apodemus mice were consumed, followed by Microtus voles, and to a lesser extent, Arvicola water voles, Talpa moles and Crocidura shrews. Stoats ate the fruits of at least five plant species and defecated the seeds intact, thus acting as potential dispersers via endozoochory. Presumably, this diverse diet was favoured by the biogeographical complexity of the study area, where the Eurosiberian and Mediterranean regions meet, its wide elevation range and heterogeneous landscape. However, the highest percentages of biomass ingested corresponded to small mammals in any season and at any elevation, although they were slightly lower in spring-summer and in the upper valley.