A longitudinal twin study was conducted to determine whether personality traits with significant heritability in adolescence remain so in adulthood. A subsample of a group of twins who had been administered in Minnesota Multiphasic Personality Inventory and the California Psychological Inventory in adolescence was readministered the same two inventories 12 years later. The subsample was found to be representative of the sample from which it was drawn. Different patterns of significantly heritability were found for the two ages studied: Several personality traits demonstrated evidence of significant heritability in either adolescence or adulthood, while others demonstrated evidence of significant heritability at both ages. In addition, genetic influences on the change from adolescence to adulthood were found for several personality traits. The results raise important questions for the interpretation of twin research and the understanding of genetic influences on personality development.