The Minnesota Study of Twins Reared Apart has conducted comprehensive medical and psychological assessments of monozygotic (N = 49) and dizygotic (N = 25) twin pairs, separated early in life (average age of separation = 0.3 and 1.1 years, respectively) and reared apart during the formative years (average age of reunion = 30.3 and 37.2 years, respectively). The twins are administered two special mental ability batteries. The Hawaii Battery (H-B), supplemented by several Educational Testing Service tests, is administered toward the beginning of the assessment week. The Comprehensive Ability Battery (CAB) is administered toward the end of the assessment week. All data are age- and sex-corrected. The average MZA and DZA intraclass correlations for the 15 H-B subtests were 0.45 and 0.34, respectively, and the average MZA and DZA intraclass correlations for the 13 subtests of the CAB were 0.48 and 0.35, respectively. Biometric model-fitting of these data indicate an average heritability of about 0.50. Data for groups of subtests in the Verbal, Spatial, Perceptual Speed and Accuracy and Memory domains were compared to a meta-analysis of the special mental ability findings in the ordinary twin literature. The Spatial domain appears to yield the highest and the Memory domain the lowest heritabilities.