This study investigated the developmental trajectories and interrelationships of mathematics-related achievement emotions and arithmetic fluency from first to third grade, and the effects of these on third grade mathematics performance. Participants were 232 Norwegian students. Students’ emotions and arithmetic fluency were measured four times and mathematics performance once. Applying latent growth curve modeling, developmental patterns of decreasing enjoyment and increasing boredom were observed over time. The mean level of enjoyment remained fairly high, and of both boredom and anxiety quite low. Individual differences were observed in both the initial levels and development of all emotions and arithmetic fluency, indicating differences in developmental trajectories. Only the initial levels and rate of change in arithmetic fluency predicted mathematics performance at the third grade.