We compared the structure and the content of mothers' and fathers' infant-directed speech as a function of infant birth order. Seven first-born and four second-born infants were video-recorded during their natural dyadic interactions with their mothers and fathers at home from the second to the sixth month after birth at 15-day intervals. Microanalysis of parental speech according to infant birth order revealed structural variations, qualitative and quantitative differences that indicate patterns of complementarity and, stability with mother and father. These results are discussed in relation to the psychobiological theory of intersubjectivity and of the complementary, intimate and trusted company and care both parents give to the developing personality of infants.