The author used qualitative research methods to explore an 8th-grade mathematics teacher's personal definition of cooperative learning and the enactment of cooperative learning in his classroom according to that definition. Data collection involved interviews and classroom observations. The author used coding schemes and descriptive statistics for data reduction and analysis. Constructivist psychology provided the theoretical groundwork for conclusions based on consistency across interview and observational data. Results revealed that while the teacher implemented a research-based model of cooperative-learning instruction, he adapted the model for use in his classroom. Results also identified the teacher's prior experience and teaching context as factors that influenced his implementation of cooperative-learning instruction.