This study investigates similarities and differences in the interactions of infants with mothers and fathers, and examines the structural characteristics of social interaction between infants and fathers. Data are based on analyses of videotapes showing face-to-face interaction of five healthy first born infants with their mothers and fathers. The interaction of infants with fathers as well as mothers is a reciprocal and jointly regulated process. Differences between mother-infant and father-infant interaction exist in the quality of regulation and in the temporal structure and content of games parents and infants play. Mothers and infants engaged in more verbal play while fathers and infants played more rhythmic tapping games. These differences provide the anlage for different functional tracks of development and together foster the development of a wider range of social skills than if only one pattern were available. (Author/MS)