The present study investigated the temperament model of Behavioral Inhibition/Activation Systems (BIS/BAS) conceptualized in Gray's Reinforcement Sensitivity Theory with a Japanese sample. In Study 1, a Japanese version of BIS/BAS scales was developed and administered to 446 university students. Results showed that the BIS/BAS scales had sufficient internal consistency and validity. Study 2 examined the relationship between the BIS/BAS scales and biological foundations of individual differences, using the twin method. Twins recruited for Keio Twin Project completed a questionnaire. Results of human behavior genetic analyses of the data from 293 pairs revealed that approximately 30% of observed individual differences in BIS and BAS could be attributed to independent genetic factors. These findings suggested that the BIS and BAS scales had biological validity as personality measures.