In this study, we explore oral and written work (plays and rap songs) of students in a sixth-grade all African-American urban science class to reveal ways affective and social aspects are intertwined with students' cognition. We interpret students' work in terms of the meeting of various genres brought by the students and teachers to the classroom. Students bring youth genres, classroom genres that they have constructed from previous schooling, and perhaps their own science genres. Teachers bring their favored classroom and science genres. We show how students' affective reactions were an integral part of their constructed scientific knowledge. Their knowledge building emerged as a social process involving a range of transactions among students and between students and teacher, some transactions being relatively smooth and others having more friction. Along with their developing science genre, students portrayed elements of classroom genres that did not exist in the classroom genre that the teacher sought to bring to the class. Students' work offered us a glimpse of students' interpretations of gender dynamics in their classrooms. Gender also was related to the particular ways that students in that class included disagreement in their developing science genre. © 2002 Wiley Periodicals, Inc. J Res Sci Teach 39: 579–605, 2002