Reports that infants imitate the vocal pitch characteristics of adult caregivers (e.g. Lewis, 1936/1951) include Lieberman's (1967; Lieberman, Ryalls & Rabson, 1982) claim that infants differentially adjust their vocal pitch or fundamental frequency (f0) towards that of their caregivers, resulting in higher mean pitch when interacting with mothers than when interacting with fathers. However, a recent cross-sectional study of infants at ages 0;8 to 0;9 and 1;0 failed to find evidence of differential pitch adjustment toward male and female caregivers (Siegel, Cooper, Morgan & Brennesie-Sarshad, 1990). A more sensitive test of Lieberman's claims would be to use a longitudinal design, with spontaneous recording sessions repeated over many months. The current study presents data from a longitudinal case study of an infant recorded at ages 0;3, 0;7, 0;10, 1;3 and 1;5 interacting with each of her parents in spontaneous play sessions and in isolated play. The infant in our study did not demonstrate significant adjustment of her vocal pitch in the direction of either parent. However, we did find evidence for consistent adjustment by the parents, in accord with the literature on infant-directed speech and mother–infant dyadic interactions, which suggest that the parents adjusted their behaviour to suit the infant more than vice versa.