The early noncry vocalizations of infants are salient social signals. Caregivers spontaneously respond to 30%-50% of these sounds, and their responsiveness to infants' prelinguistic noncry vocalizations facilitates the development of phonology and speech. Have infants learned that their vocalizations influence the behavior of social partners? If they have, infants should show an extinction burst in vocalizing when adults temporarily stop responding to infant vocalizations. Thirty-eight 5-month-olds were tested in the still-face paradigm with an unfamiliar adult. When the adult assumed a still face, infants showed an extinction burst. Thus, 5-month-olds have learned the social efficacy of their vocalizations on caregivers' behavior. Furthermore, the magnitude of 5-month infants' extinction bursts predicted their language comprehension at 13 months.