Adult wood mice were tested in cages for their responses to single newborn pups and mothers of newborn litters were tested for their ability to protect litters from other adults. There was no difference in infanticidal tendencies of virgin male or females, however, males housed with perforate females were less infanticidal than those with non-perforate females. Mothers of litters were unable to prevent infanticidal intruder males entering the nest but could exclude females. These data are discussed with reference to the social organization and studies on population regulation of wood mice.