The contribution of heredity to scores on a digit span intelligence test, Jensen's Memory for Numbers, was estimated with a standard heritability formula. The test measures level I mental ability--the capacity to store and recall, but not ability to elaborate or manipulate stimuli. Subjects were 35 monozygotic (MZ) twins and 35 same-sex dizygotic (DZ) twins, ages 11-18 who were raised together. Twin intraclass correlations derived from the analysis of covariance of the test scores were .716 (MZ) and .497 (DZ). The broad heritability was .437 or .52 when corrected for unreliability. Height and weight measurements were also applied to the heritability formula, to compare results of these known traits to a little known trait--memory span; heritability values were .958 (height) and .730 (weight), which supported previous research. Analysis of abstract reasoning or level II ability, as measured by Raven's Standard Progressive Matrices Test, resulted in a heritability value of .897. This was a high value for mental test data. The MZ data showed no evidence of a first-order genotype by environment interaction. (Author/CP)