Mating behaviour and its relation to social structure was studied in two colonies of wild caught House mice (Mus musculus) housed in semi-natural environments. In both colonies, one male was initially dominant to (chased) a number of subordinate males, and mated significantly more often than subordinate males. In time, younger males deposed the dominant male and set up defended areas in the observation room. Under this territorial social structure, only males who possessed defended areas mated. Females moved freely throughout the observation room, and chose high quality territories in which to deliver their litters. Females mated significantly more often with a) males in whose territories they delivered their litters and b) males who defended‘high quality’territories.