In this study a mother's instinctive accommodations of vocal fundamental frequency (f0) of infant-directed speech to two different infants was explored. Maternal speech directed to individual 3-mo.-old fraternal twin-infants was subjected to acoustic analysis. Natural samples of infant-directed speech were recorded at home. There were differences in the rate of infants' vocal responses. The mother changed her f0 and patterns of intonation contour when she spoke to each infant. When she spoke to the infant whose vocal response was less frequent than the other infant, she used a higher mean f0 and a rising intonation contour more than when she spoke to the other infant. The result suggested that the mother's speech characteristic is not inflexible and that the mother may use a higher f0 and rising contour as a strategy to elicit an infant's less frequent vocal response.