The aim of the present study is to learn what kind of speech is effective to elicit and maintain prelinguistic infants' vocal responses. Information about the parent-infant interaction is beneficial for understanding how to communicate with children who have communication disorders. We compared the prosodic features in Japanese mothers' and fathers' speech directed to prelinguistic infants, and differences in prelinguistic infants' use of the fundamental frequency (f_0) and the rate of their vocal responses toward their mothers and fathers. Our findings were as follows: (1) When fathers speak to infants, they use a higher f_0, more exaggerated f_0, a shorter duration, and an intonation contour that rises more than when they speak to adults. These characteristics of prosody are similar to mothers' infant-directed speech. (2) The three 7-month-old infants observed did not use a different f_0 when they interacted with their mothers and with their fathers. (3) The infants' rate of vocal responses in mother-infant interactions was different from that in father-infant interactions. The infants tended to respond to their mothers with more vocalizations than to their fathers.