ABSTRACT Previous behavior-genetic research on personality has been almost exclusively based on self-report questionnaire measures. The purpose of this research was to measure personality constructs via self- and peer reports on the items of the NEO Five-Factor Inventory (Costa & McCrae, 1989). The sample included 660 monozygotic and 200 same sex and 104 opposite sex dizygotic twin pairs. We collected self- and two independent peer reports for each of the twins. Our analysis of self-report data replicates earlier findings of a substantial genetic influence on the Big Five (h2= .42 to .56). We also found this influence for peer reports. Our results validate findings based solely on self-reports. However, estimates of genetic contributions to phenotypic variance were substantially higher when based on peer reports (h2= .51 to .81) or self- and peer reports (h2= .66 to .79) because these data allowed us to separate error variance from variance due to nonshared environmental in-fluences. Correlations between self- and peer reports reflected the same genetic influences to a much higher extent than identical environmental effects.