Wood mice Apodemus sylvaticus and house mice Mus domesticus are two of the commonest and most intensively studied British mammals. However, relative to the vast literature on non-commensal and laboratory house mice (Berry, 1981, 1991), and woodland wood mice (Flowerdew, Gurnell & Gipps, 1985; Flowerdew, 1991) little is known of the ecology of either species in and around farm buildings. Farm buildings and their surroundings are particularly important for house mice, which are a major stored-product pest (Meehan, 1984). In Britain they live largely indoors but also make limited use of hedges and fields in the summer (Rowe & Swinney, 1977; Montgomery & Dowie, 1993). Wood mice are very abundant on agricultural land, and occasionally use buildings (Green, 1979; Montgomery & Dowie, 1993; Tew & Macdonald, 1993).