When science undergraduates begin their upper-division coursework, their declaration of major becomes more concrete and meaningful as they have opportunities to interact more deeply with the community of their chosen discipline. In the process of completing a major, students transition their identity towards being a member of their field. In Wenger's community of practice framework, community membership is built on alignment of common goals, participation in social interactions, and perception of belonging in the community. But what does it mean to be a chemist or physicist from the students' perspective? In this study, we examine junior-level chemistry and physics majors' ideas about their science identity through semi-structured interviews and prompted reflective journals. We compare and contrast how chemistry and physics students negotiate their identity as members in their disciplinary field in terms of practice, qualifications, attitude, and in relation to other STEM communities.