Observers watched videotaped face-to-face mother-infant and stranger-infant interactions of 12 infants at 2, 4, or 6 months of age. Half of the observers saw each mother paired with her own infant and another infant of the same age (mother tapes) and half saw each infant paired with his or her mother and with a stranger (infant tapes). Observers were asked to judge which was the mother-infant dyad in each pair. Observers' accuracy improved as infants aged and was above chance for both mother and infant tapes when infants were 6 months. Differences between mother-infant and stranger-infant dyadic communication patterns also emerged as the infants aged. At 6 months, mother-infant dyads had more symmetrical communication and less asymmetrical communication than stranger-infant dyads. (Contains 2 tables and 4 figures.)