Social interaction and spatial distribution among male mice (Mus musculus) were observed. The enclosure was divided into three compartments, left, right and center. Each side compartment contained two males for 3 d and then the doors that led to the center compartment were opened for 7 d. The dominant male was removed 3 d after the opening day. The results indicate that the dominant male monopolized its own territory including the compartment where it had dwelt before opening the doors. The other subordinates huddled together in the opposite compartment. The dominant male patrolled more frequendy than subordinates and sometimes attacked subordinates. One of the subordinates rapidly became dominant after the removal of the dominant male. The secondary dominant male monopolized the familiar area and attacked subordinates, as the dominant male had done.