In this paper the differential participation of men and women in scientific communities of practice was examined in two graduate science departments. Data from a survey questionnaire and interviews with graduate students and faculty members were used to examine students' level of participation in their community of practice. Within each department, vital areas were identified which either limited or facilitated students' participation. Collaboration among the members of research teams, collegiality among research groups, and diversity within the academic and student bodies facilitated students' participation at various levels. In contrast, competition, lack of collaboration, and lack of collegiality and diversity, limited students' participation in their communities of practice.