We examine whether integrating men and women in a traditionally male-dominated environment can change men's attitudes about mixed-gender productivity, gender roles, and gender identity. Our context is the military in Norway, where we randomly assigned female recruits to some squads but not others during boot camp. We find that living and working with women for eight weeks causes men to have more egalitarian attitudes. There is a 14 percentage point higher fraction of men who think mixed-gender teams perform as well or better than same-gender teams, an 8 percentage point increase in men who think household work should be shared equally, and a 14 percentage point increase in men who do not completely disavow feminine traits. Moreover, men in mixed-gender teams are more likely to choose military occupations immediately after boot camp that have a higher fraction of women in them. But these effects do not persist once treatment stops. Treated men’s attitudes converge to those of the controls in a six-month follow-up survey, and there is no long-term effect on choosing fields of study, occupations, or workplaces with a higher fraction of women after military service ends. Contrary to the predictions of many policy makers, we do not find that integrating women into squads hurt male recruits’ performance or satisfaction with service, either during boot camp or their subsequent military assignment. These findings provide evidence that even in a highly gender-skewed environment, gender stereotypes are malleable and can be altered by integrating members of the opposite sex. But they also suggest that without continuing intensive exposure, effects are unlikely to persist.