This paper discusses the relationship between aggressive behaviour and sexual maturity, prepubertal castration and wounding in male Bank voles, Clethrionomys glareolus. In neutral arena tests in the laboratory, sexually mature males exhibited more aggressive components of behaviour than immature males or than adult males that had been castrated prepubertally. Mature males also exhibited fewer amicable components than the immature or castrated males. During several months residence in large outdoor enclosures, mature males consistently had more fresh skin wounds (inflicted by other voles) than did either immature or castrated males. The relevance of these results to the population ecology of Bank voles is discussed.