Previous research has indicated that 4-month-old human infants who exhibit high degrees of motor activity and negative affect in response to the presentation of unfamiliar auditory and visual stimuli are likely to display behavioral inhibition as toddlers, while 4-month-old infants who display high degrees of motor activity and positive affect in response to the same stimuli are likely to be behaviorally exuberant toddlers. The present study examined baseline and fear-potentiated startle eyeblink responses during a stranger-approach paradigm at age 9 months in a group of infants, some of whom displayed high motor activity and negative affect and some of whom displayed high motor activity and positive affect at 4 months. The analyses revealed that the high motor/high negative group of infants exhibited a significantly greater increase in fear-potentiated startle amplitude at 9 months compared with the high motor/high positive group. There were no differences among groups of infants on baseline startle responses. These findings suggest that the origins of behavioral inhibition in early childhood may be linked to a low threshold for arousal in forebrain limbic areas.