Large series of Clethrionomys glareolus (Schrober) were collected in three Scottish localities and used to study the variability in size and in several other external, cranial and dental characters in relation to age, season and year. These collections were then compared with samples of local island races, and collections from the south-east of England. It was concluded that most of the regional differences in skull-proportions are manifestations of the differences in overall size and that the number of independent characters distinguishing the island races is very small. The results are considered not to support the theory that the four described island races are relict in origin.