Sex differences in mathematical achievement and spatial visualization skill were examined in a sample of 724 Norwegian sixth-grade students. Boys had significantly higher mean mathematics scores than girls. Significant sex differences favouring boys were found in the subsamples of most difficult tasks, but not in the subsamples of easiest tasks. No significant sex difference in spatial visualization was found. The hypothesis that boys' superior achievement in mathematics is due to a superior ability in spatial visualization was not supported. Although the effect of spatial visualization on mathematical achievement increased significantly up to a certain level of mathematics task difficulty, the hypothesis that the effect of spatial visualization on mathematical achievement increases with increasing task difficulty was not fully supported. With increasing mathematics task difficulty, it is hypothesized that boys, more than girls, will benefit from spatial visualization. This hypothesis was not supported by the present elementary school data.