The social organization, nightly movements and nesting behaviour of Wood mice. Apodemus sylvaticus (L.), were studied by radio-tracking 35 individuals in a deciduous wood in north-east Scotland. The radios, which were mounted on collars and weighed 2·4 g, were only attached to mice weighing more than 20 g. During the breeding season (mid-March to December), when all the mice tracked were reproductively active, male ranges were on average 3·6 times as big (5110 m2) as those of females (1424 m2); male ranges overlapped considerably whilst females probably maintained ranges largely exclusive of one another. The ranges of males overlapped those of females apparently at random, each male's range encompassing parts of about five to ten females' ranges. Individuals' ranges changed position with time, although no total displacements were noted. No long-term bisexual associations were recorded. Nests appeared to be located at random within ranges, and nest sites were changed frequently. In summer, mice nested solitarily, whilst in winter they nested in groups of up to at least three. Typically, mice moved extensively over their ranges each night, males moving further between successive radio-fixes than females, and having fewer activity loci. Throughout the year, mice occasionally left their normal home ranges on excursions, sometimes over 100 m in length.