This study investigated the phenotypic, genetic, and environmental relationships between Big Five personality traits and temperamental traits on the basis of the regulative theory of temperament (RTT) using a multitrait-multimethod twin dataset. This allowed us to test specific hypotheses of the five factor theory (FFT) and the RTT. The Big Five personality factors were measured with the Neuroticism-Extraversion-Openness Five-Factor Inventory (NEO-FFI). The six regulatory temperamental traits were captured by the Formal Characteristics of Behavior-Temperament Inventory (FCB-TI). We analyzed self-reports as well as averaged peer ratings from 737 monozygotic and 395 dizygotic twins. Results showed phenotypic links between Big Five and RTT traits that could be attributed mainly to genetic factors. Genetic influences on the variance in Big Five personality traits did not account for the vast majority of genetic variance in all temperamental traits of the RTT contradicting the hypothesis of the FFT that the Big Five exhaustively cover basic temperamental traits. In line with the RTT, the FCB-TI scales showed large genetic links to Neuroticism and Extraversion and rather small links to Openness, Agreeableness, and Conscientiousness, supporting the idea of a differentiation between regulative and integrative aspects of personality.