The significance of three mothers' speech for their infants' language development is considered in a continuing longitudinal study. The study began when the children (two females and one male) were 5 and 6 months of age and will continue until the subjects are 3 years old. In the speech data reported the children were from 6 to 18 months of age. During individual one-hour taping sessions every 3 or 4 weeks, the mother and experimenter talk for about 15 minutes. During the rest of the hour, mothers play and talk naturally with their children. Children's speech appears to correspond to mothers' speech, in terms of complexity of imitated responses and speech patterns. In addition, data show that mothers modify their speech on the basis of linguistic cues from their children. (BRT)