Genetic analysis of data from 2,680 adult Australian twin pairs demonstrated significant genetic contributions to variation in scores on the Harm Avoidance, Novelty Seeking, and Reward Dependence scales of Cloninger's Tridimensional Personality Questionnaire (TPQ), accounting for between 54% and 61% of the stable variation in these traits. Multivariate genetic triangular decomposition models were fitted to determine the extent to which the TPQ assesses the same dimensions of heritable variation as the revised Eysenck Personality Questionnaire. These analyses demonstrated that the personality systems of Eysenck and Cloninger are not simply alternative descriptions of the same dimensions of personality, but rather each provide incomplete descriptions of the structure of heritable personality differences.