A comprehensive working memory battery and tests of mathematical skills were administered to 90 children--41 in Year 1 (5-6 years of age) and 49 in Year 3 (7-8 years of age). Working memory could explain statistically significant variance in number writing, magnitude judgment, and single-digit arithmetic, but the different components of working memory had different relationships with the different skills. Visual-spatial sketchpad (VSSP) functioning predicted unique variance in magnitude judgments and number writing. Central executive functioning explained unique variance in the addition accuracy of Year 1 children. The unique variance explained in Year 3 multiplication explained by phonological loop functioning just missed conventional levels of significance (p = 0.06). The results are consistent with the VSSP having a role in the development of number writing and magnitude judgments but a lesser role in early arithmetic. (Contains 8 tables.)