Does the gender composition of a workplace influence the ways in which team members are evaluated?  Can it have long-term effects on labor market outcomes?  We conducted a multi-year field experiment with a top-10 undergraduate accounting program that was designed to identify the effect of work team gender composition on men and women’s leadership, performance, and long-term labor market outcomes.  Using survey, laboratory, and administrative data, we find that women who were randomly assigned to be the lone women on a work team were rated less authoritative and less competent than women on majority-women work teams.  In a team lab task, they were less able to influence the outcome of the task.  And while their grades were not affected, women’s long-term professional plans are affected by the gender composition of their group.  Our findings have profound implications for how workplaces approach team assignment and the evaluation of women in male-dominated settings.