Five-month-old infants' vocalization when alone was investigated. Several researchers have reported observing that young infants vocalize in comfortable states without any response from others. As is implied by episodic reports in previous studies, it is possible that infants vocalize to play with their own vocal sound. Producing and listening to their vocalizations as sound play could be an early emergence of musical behavior. A hypothesis of this study is that infants vocalize when alone in a comfortable condition for the purpose of listening to their sound, and not to elicit responses from others. Three experimental conditions were conducted in the infants' homes: the response condition, where the mother responded to the infant; the no-response condition, where the infant was left alone in the room and began spontaneous sound production; and the no-response-amplified condition, where the second condition was accompanied by amplified immediate sound feedback. The results suggested that the ratio of sound was higher in the no-response condition, and even higher in the no-response-amplified condition. In both conditions where the infants were alone, repetition of the same phrase was increased. The possible implications of infant vocalization simply to listen to their sounds are discussed. (Contains 2 tables and 2 figures.)