Phenotypic, genetic, and environmental correlations between various reaction time measures and intelligence were examined in a sample of six-year-old twin children (N = 530 individuals). Univariate genetic analyses conducted on the same-sex pairs (101 monozygotic (MZ) pairs and 132 same-sex dizygotic (DZ) pairs) demonstrated that the intelligence measure and four of the seven reaction time measures had a genetic component (ranging from 44% to 76%). At the phenotypic level, half of the reaction time measures had significant negative correlations with the intelligence measure. Bivariate genetic analyses revealed that only two of the observed phenotypic correlations could be explained by common genetic factors and that the remaining correlations were better explained by common environmental factors.