To gain insight in developmental trajectories of motivation during upper primary school, the present study focused on how different aspects of students' motivation, i.e., task-orientation, self-efficacy, and school investment develop from grade three to six of primary school and how these developments differ for boys and girls, and students with different ethnic or social backgrounds. Furthermore the longitudinal relation between motivation and achievement in reading comprehension was examined. A total of 722 students completed questionnaires during five measurements. Latent growth curve analyses were performed. Results showed a negative development in task-orientation, self-efficacy remained relatively stable and school investment increased over time, but there were considerable differences in developments across different groups of students. Regardless of gender and background, however, developments in these aspects of motivation were substantially positively related to developments in achievement, beyond what can be explained by cognitive ability and background characteristics. (Contains 6 tables and 4 figures.)