Olfactory bulbectomized and control male CF-1 strain mice were housed in separate, large, seminatural environments over a 30-day observation period. Control mice engaged in vigorous fighting behavior before a dominant animal emerged. The alpha mouse established the floor area as its territory and vigorously attacked all other mice that ventured from the second tier of the enclosure to the floor. Lower ranking mice had numerous body scars, were much less aggressive, and engaged in a variety of nonaggressive social interactions. In contrast, bulbectomized mice did not fight and roamed freely through all areas of the environment. These mice largely ignored one another, engaged in little or no social behavior, and had no body scars. When two anosmic mice came into physical contact, they appeared startled and moved away from one another. These results, together with those of prior studies, indicate that olfaction plays a critical role in virtually all aspects of social behavior in male mice.