Measurements of four skull and five external characters were made on each of 277 specimens of Apodemus sylvaticus (L.) from the two Isles of Scilly (Tresco and St Mary's) on which it occurs and from six localities in mainland Cornwall. The skull characters were greatest length of skull, maxillary tooth row, cranial breadth and length of palatal foramen. Lengths of the pectoral stripe, tail, hindfoot and ear were obtained, as well as the weight of each animal. The age of each individual was assessed by means of a tooth wear character and a regression adjustment was used to bring all the characters to a uniform age class. The adjusted and unadjusted means have been calculated of each character for every locality. The sexes were separate throughout the analysis.
The island mice are slightly larger than those from the mainland, the latter forming a fairly homogeneous group. In addition to overall size the most distinctive sign for the Tresco mice is the particularly small pectoral stripe. The St Mary's mice have a long tooth row, large pectoral stripe and large palatal foramen. The females were usually slightly smaller than the males.
A canonical analysis was made in an attempt to account for as much variation as possible between groups using a limited number of linear combinations of the original measurements. The greater part of the variance was contained in the first two canonical variates. From these, the populations separated into three groups, i.e. the mainland group, the mice from Tresco and those from St Mary's. The two Scilly Island populations are as distinct from each other as they are from the mainland populations. Separate introductions from different sources and, possibly, slightly different genetic composition of the original immigrants may have accounted, in part, for the divergence shown by these two island stocks. It is not proposed that the island races should be given subspecific status.