The literature on the use of problem-based learning in K-12 settings has traditionally focused on gifted and average students. However, mainstreaming is placing increasing numbers of students with special needs in general education classrooms. This case study examined how members of a small group in a mainstreamed seventh grade science class interacted with and supported each other as they engaged in a problem-based learning (PBL) unit. The group included one mainstreamed and two average students. We used conversation analysis and coding to analyze interview and video data of all 10 class sessions. Results indicated that each group member filled a unique role--group manager, task guidance provider, and task performer--and helped each other overcome individual difficulties. Results suggest that mainstreamed groups have the potential to effectively engage in PBL, and that PBL may increase the motivation and social confidence of students with special needs. We suggest types of scaffolds that could support mainstreamed students during PBL units. (Contains 1 figure.)