Male albino and wild house mice were reared from weaning (at 21 days) either to 37 or 56 days of age in: (1) isolation, (2) all-male groups of four, or (3) all-male groups of four in cohabitation with an adult female. At 37 days the cohabiting albino males exhibited larger testes and epididymides than animals in the other two situations. Similarly, they had larger seminiferous tubules and were significantly more mature (by an arbitrary rating system) than the isolated males or the males in all-male groups. Wild males reared in isolation or in cohabitation were reproductively superior in essentially the same fashion to males in all-male groups. Cohabiting wild males, however, were significantly more mature than wild males in either of the other two conditions.