This study describes how a physical education teacher collaborated with a second-grade teacher to plan and implement an interdisciplinary unit, and it identifies factors that contributed to the teachers' actual collaboration. One accomplished elementary physical education teacher, one experienced second-grade classroom teacher, and 35 students from two second-grade classes voluntarily participated in this study. The data were collected by audiotaping the two planning sessions, videotaping eight integrated lessons taught by the physical education teacher and three integrated lessons taught by the classroom teacher, transcribing the taped lessons, and interviewing the teachers. The findings indicated that the teachers' collaborative planning focused on providing students with integrated and relevant learning experiences. Throughout the collaboration, the two teachers shared leadership roles and teaching responsibilities. The teachers attributed their effective interdisciplinary teaching to their long-term collaborative working experiences, common teaching philosophy, and mutual respect and trust.