Although a growing literature has documented the effectiveness of informal group work during both lecture- and studio-style science and math classes, virtually no data exist on which students are collaborating with each other. We wanted to find out how students self-sort when they know that they will be asked to work in small groups on graded activities. In a large-enrollment introductory biology course for majors that emphasized intensive peer interaction, we documented which students worked together on each of five in-class paper and pencil exercises that were scheduled throughout the term. We used pair-wise logistic regression models to assess the likelihood that students collaborated based on their demographic characteristics, socioeconomic status, and academic performance. In almost all five activities, students self-sorted by ethnicity and gender. Interestingly, by the end of the quarter students who were doing well in the course began collaborating with each other.