This study explored the development of children's representations of the affiliative structure of their peer group during primary school. Subjects were 467 boys and girls in first through sixth grade, who were assessed using a standard sociometric procedure and asked to give their perception of the existing networks in their peer group. Variance analysis was conducted by gender and grade level. Results showed that across all age levels, both girls and boys perceived themselves to be mainly affiliated with individuals of their own gender and that this pattern increased as a function of grade level. Exhaustivity and accuracy of the perceptions do improve with age, girls surpassing boys. In addition, both sexes appeared to show a better perception of the peer networks of their own gender, yet this pattern fluctuated with age. Results are discussed in light of current research on gender segregation, gender differences, and cognitive processes involved in social categorization. (Author/MM)