Forty-eight boys and 48 girls aged between 32 and 64 months were tested for stereotyping of sex role using a toy-preference technique. Scores for overt and covert measures of sexism and awareness of the dimension of sex-role stereotyping were also obtained from both parents of each child, as well as information on the target child's siblings. Analysis indicated the developmental nature of stereotyping of sex role in both boys and girls and the surprising finding that older siblings are much more important in the development of such stereotyping than are sexist parents. The influence of the child upon parental attitudes is also apparent. These findings and their interrelationships are discussed, suggesting new directions for research. © 1980 Plenum Publishing Corporation.