The mechanisms underlying performance on the Star Counting Test (SCT) and its nomothetic span were investigated along with the relationships between working memory capacity, fluid intelligence (Gf), speed, and school achievement. The SCT is an attention test for children that requires the alternation of forward and backward counting. The test is based on A. D. Baddeley and G. J. Hitch's (1974) model of working memory in conjunction with D. A. Norman and T. Shallice's (1986) theory of central executive functioning. Tests were administered to 1,122 boys and 1100 girls in 4th grade from 111 Dutch schools. The SCT required flexible alternation, counting speed, and sustained effort. Factor analysis showed that the SCT forms one factor with other indicators of working memory capacity. There was also a strong association between working memory capacity and Gf. The two clearly differ, however, in their relation to speed.