The relationship between the two species of field mice, Apodemus sylvaticus (Linnaeus) and A. flavicollis (Melchior), often close neighbors in the same habitat, has long intrigued naturalists. In Britain, adults of the two species have frequently been paired to see if hybrid offspring could be obtained, but always with negative results. For example, Cranbrook (personal communication) has paired males and females of the two species from parkland in Suffolk, and maintained them together, but reports that they show no sexual response to one another. On the continent of Europe, Zimmermann (1957) and Saint-Giron (1962) equally have failed to get these species to interbreed. On the other hand, a number of workers (Dalimier, 1952, 1955; Frechkop, 1955) maintain that they are conspecific. The taxonomy of the genus Apodemnus presents a number of difficulties because of the close resemblance between species in the characters of skull and pelage usually employed for separation. Ellerman and Morrison-Scott (1951) were obliged to distribute the majority of races in a somewhat arbitrary manner between sylvaticus, average smaller skull, and flavicollis, average larger skull. . This system resulted in many of the large races of field mice, such as those from the islands of St. Kilda, Rhum, and Fair Isle, being classified as subspecies of A. flavicollis. Rather more is now known of the biology of these island