Results from 2 twin studies of aging differ concerning the heritability of memory performance in adulthood. Heritability estimates from the Minnesota Twin Study of Adult Development and Aging (MTSADA; D. Finkel & M. McGue, 1993) are larger than estimates from the Swedish data (N. L. Pedersen, R. Plomin, J. R. Nesselroade, & G. E. McClearn, 1992). Memory data were available from MTSADA on a sample of 112 monozygotic (MZ) twin pairs and 111 dizygotic (DZ) twin pairs ages 27 to 88 years and from the Swedish study on a sample of 86 MZ and 110 DZ twin pairs who were reared together and were ages 27 to 85 years. Univariate and multivariate behavior genetic analyses were conducted to determine the significance and source of differences in heritability across studies. No significant age differences were found in either study. Study differences were found at the level of specific memory measures, but not at the level of a general memory factor.