Discriminant analysis was used to distinguish statistically between the comfort-state vocalizations uttered by Japanese infants following 5 different types of pitch contours of maternal speech. Ontogenetic changes of their vocal behaviors were investigated during the first 5 months of life. Structural variability in infant vocalizations across variants of maternal speech was found to be characterized by a set of quantifiable physical parameters. However, infant's age when a vocalization was recorded was not an important contributor. Successful attempts at cross-validation, in which the discriminant profiles derived from one sample of vocalizations were used to classify a second set of vocalizations, indicated that the result obtained was not an artifact of using the same data set to derive the profiles to test reclassification accuracy. Proportion of cross-validated vocalizations that were misclassified decreased as age increased. The results of the present study demonstrate that a statistically significant relation exists between acoustic features of maternal speech and those of following infant vocalizations, and that such a relation strengthens over age.