A total of 120 field-mice from Fair Isle (Apodemus sylvaticus fridariensis (Kinn.)) have been examined. Fifty-eight of these were collected in 1959 and the remainder were from various museums. Occipito-nasal lengths, sex, tooth wear and weights have been recorded and used in the reconstruction of information on the life-history. Breeding appears to start in February and terminate in late summer. Most of the present trappings were in the vicinity of the hostel and in the Duttfield-Vaadal area. Mice were never very numerous and appeared to frequent dry-stone dykes and the hostel buildings. The Fair Isle mouse is larger than the mainland mouse and slightly smaller than the St. Kilda mouse.