We examined differences in brain electrical activity (EEG), heart rate (EKG), heart rate variability, and behavior among 40 young women who were selected for high (HI) and low (LO) self-ratings of shyness and sociability. EEG and EKG were continuously recorded for 6 min from each subject, just prior to an expected novel social encounter. Each subject was also observed unobtrusively for 5 min during the social encounter. Analyses indicated that the pattern of frontal EEG asymmetry was related to sociability, but not to shyness. LOSOCIABLE subjects displayed greater relative right mid-frontal activation while HISOCIABLE subjects displayed greater relative left mid-frontal activation. A significant shy x sociable interaction was found for parietal asymmetry and for measures of heart rate and heart rate variability. LOSHY/HISOCIABLE subjects displayed greater relative right parietal activation while LOSHY/LOSOCIABLE subjects displayed greater relative left parietal activation. And HISHY/HISOCIABLE subjects displayed a significantly higher and more stable (less variable) heart rate than HISHY/LOSOCIABLE and LOSHY/HISOCIABLE subjects. Analyses also indicated that LOSOCIABLE subjects displayed significantly more signs of nonverbal social avoidance as compared with HISOCIABLE subjects. The present findings suggest that shyness and sociability may be subserved by different neurophysiological systems.