Although previous studies have examined the genetic and environmental influences upon general intelligence and specific cognitive abilities in school-age children, few studies have examined elementary cognitive tasks in this population. The current study included 149 MZ and 138 same-sex DZ twin pairs who participated in the Western Reserve Twin Project. Thirty measures from the Cognitive Abilities Test (CAT; Detterman, 1986) were studied. Results indicate that (1) these measures are reliable indicators of general intelligence in children and (2) the structure of genetic and environmental influences varies across measures. These results not only indicate that elementary cognitive tasks display heterogeneous genetic and environmental effects, but also may demonstrate that individual differences in biologically based processes are not necessarily due to genetic variance.