This longitudinal and naturalistic study investigated fathers' and infants' emotional expressions during free infant-father "protoconversation" and the preceding/following pauses. Eleven infant-father dyads were observed during spontaneous interactions at home, from the second to the sixth month after birth. Micro-analysis of infant and paternal emotional expressions provided evidence that: (a) in the course of infant-father "protoconversation" but not during the pauses, infants and fathers match their facial expressions of pleasure and interest, (b) in the course of father-infant "protoconversation" and pauses, infants and fathers attune their emotional intensity change, (c) infants match paternal emotions more than vice versa during "protoconversation" but not in the course of pauses. Infants "complete" paternal positive emotions more than vice versa during "protoconversation" and pauses, and (d) infant age affects the developmental curves of paternal and infant pleasure and interest. These results will be discussed in the frame of the theory of innate inter-subjectivity. (Contains 2 tables, 2 figures, and 1 note.)