The aim of this study was to examine the relationship between working memory capacity and mathematical performance measured by the national curriculum assessment in third-grade children (n = 40). The national tests concerned six subareas within mathematics. One-way ANOVA, two-tailed Pearson correlation and multiple regression analyses were conducted. The results showed that working memory could be deemed as a predictor for the overall mathematical ability. However, the significance of working memory contributions varied for the different mathematical domains assessed. Working memory contributed most to basic mathematical competencies. Algorithms were not explained significantly by working memory. The contributions of different working memory resources varied as a function of the mathematical domain, but in certain respects the variance was shared across the elements and both visuo-spatial and phonological abilities seem important for mathematic performance. We suggest that individuals' working memory capacity is important to take into consideration in learning. (Contains 4 tables.)