The present study examined the extent to which motivation contributes to the prediction of school achievement among elementary school children beyond general mental ability ("g"). The sample consisted of "N" = 1678 nine-year-old UK elementary school children who took part in the Twins Early Development Study (TEDS). Teachers provided achievement assessments according to the UK National Curriculum criteria for Mathematics, English, and Science, and pupils reported their ability self-perceptions and intrinsic values for these subjects. For all three domains, "g" proved to be the strongest, and, in the case of Science, the only predictor of school achievement. However, in Mathematics and English, children's ability self-perceptions as well as intrinsic values each contributed incrementally to the prediction of achievement beyond "g" with ability self-perceptions being a better predictor than intrinsic values. Finally, commonality analyses revealed a substantial portion of common variance in school achievement explained both by "g" and motivation. In the light of these results it is argued that the study of motivation offers valuable clues for the understanding and improvement of school achievement.