AbstractAn isolated population of the bank vole,Clethrionomys glareolus, at Loch Tay, Perthshire, which was found to have abnormally complex third upper molars in 1955‐57, was re‐examined in 1972 and found to have reverted to normal. Subfossil teeth of the field vole,Microtus agrestis, from the island of Jura, Inner Hebrides, dating back to at least 2000 BP have been compared with recent specimens from the island. They demonstrate a slight but progressive reduction in the size of the postero‐medial ridge of M1that is characteristic of certain ma