Adult listeners are capable of identifying the gender of speakers as young as 4 years old from their voice. In the absence of a clear anatomical dimorphism in the dimensions of pre-pubertal boys' and girls' vocal apparatus, the observed gender differences may reflect children's regulation of their vocal behaviour. A detailed acoustic analysis was conducted of the utterances of 34 6- to 9-year-old children, in their normal voices and also when asked explicitly to speak like a boy or a girl. Results showed statistically significant shifts in fundamental and formant frequency values towards those expected from the sex dimorphism in adult voices. Directions for future research on the role of vocal behaviours in pre-pubertal children's expression of gender are considered.