Individual differences in mathematics performance may stem from domain-general factors like working memory and intelligence. Parietal and frontal brain areas have been implicated in number processing, but the influence of such cognitive factors on brain activity during mathematics processing is not known. The relationship between brain mechanisms during symbolic and nonsymbolic number comparison and individual differences in working memory, nonverbal IQ, and mathematics achievement were investigated to determine possible associations with behavior and brain function. A number of brain areas showed correlations with working memory and number processing. For symbolic digits, working memory showed a positive relationship with brain activity in a network of bilateral parietal, temporal, and right frontal regions. For nonsymbolic dot arrays, working memory showed a negative relationship with several parietal and frontal brain areas. This relationship indicates differences between behavioral and brain function measures and points to the importance of working memory and basic number processing. (Contains 2 figures and 4 tables.)