The social organization of confined groups of wild house mice has been studied by direct observation and by indirect means. The prominent aggressive behaviour which leads to the formation of dominance-subordination relationships under certain laboratory conditions is shown to result in territory formation when suitable environmental elements are provided. Fighting is almost entirely confined to adult males, but pregnant females may also defend territories. Although minor physical injury can result from fighting, survival rates are not significantly depressed if adequate cover is provided. Previous work on fighting in this species is reviewed from an ecological viewpoint and related to the results presented.