Seventy-one females of 11 species of muroid rodents were mated with conspecific males in order to investigate the nature of copulatory plugs. The presence or absence of plugs was correlated with copulatory pattern. Four species, which display a locking pattern of copulation, did not deposit copulatory plugs. By contrast, seven nonlocking species always deposited plugs. Both plug accumulation and plug dislodgement were found in multiple ejaculation tests with the nonlocking species. The plugs of hamsters can be dislodged after few intromissions. The failure of locking species to deposit plugs appears correlated with reduction in the complement of male accessory sex glands.