The authors used a person-centered, longitudinal approach to identify and evaluate naturally occurring combinations of intrinsic and extrinsic motivations among 490 third- through fifth-grade students. Cluster analysis revealed 3 groups, characterized by high levels of both motivations (high quantity): high intrinsic motivation but low extrinsic motivation (primarily intrinsic) and low intrinsic motivation but high extrinsic motivation (primarily extrinsic). Analyses of stability and change in cluster membership indicated that the primarily intrinsic cluster was most stable (76% stability) and the high-quantity cluster most precarious (45% stability) over the course of an academic year. Students in the primarily intrinsic cluster outperformed their peers in the other 2 clusters and showed the greatest increase in achievement over time.