Measurements of ten skull characters were made on each of 352 specimens of Apodemus sylvaticus (L.) and 37 specimens of A. flavicollis (Melch.). The latter were obtained from southern England and the former from five of the Channel Islands, two localities in southern England and one in northern France. The characters were greatest length, greatest width, interorbital breadth, cranial breadth, width of foramen magnum, length of maxillary tooth row, length of palatal foramen, distance between ventro-posterior processes of pterygoids, length of mesopterygoid fossa and minimum distance between first upper molars. The ages of the skulls were ascertained by means of a tooth-wear measurement. A regression adjustment was used to bring all the characters to a uniform age class. The sexes were separate throughout the analyses. The unadjusted and adjusted means have been calculated of each character for every locality. Appreciable variation occurs between the populations examined, with the females slightly smaller than the males but following the same general pattern of variation. An analysis into canonical variates was made with a view to accounting for the largest possible part of the variation between groups using a limited number of linear combinations of the original measurements. Almost all the variance was contained in the first four canonical variates and from these it appeared that the populations separated into three groups. The first contained the mainland mice from Hampshire, Cornwall and Cap Gris Nez, the second those from the Channel Islands and the third the collection of A. flavicollis. The analysis confirms that in the British Isles A. flavicollis is distinct from A. sylvaticus and that the Channel Island mice, although larger than the mainland A. sylvaticus, probably belong to that species. Differences between populations from the Channel Islands are small, although the mice from Herm and Sark are relatively large. It is possible that the Channel Island mice could have been derived from a common stock.