This study investigated the development of children's conceptions of and reactions to cross-gender behavior in their peers. Eighty elementary-school children at tour grade levels (kindergarten and second, fourth, and sixth grades) were interviewed about their attitudes toward hypothetical peers who violated child-generated norms for sex-typed behavior in each of four categories (traits, toy s, activities, and friendship preferences). Older children reported that they would react more negatively toward cross-gender displays of activity (e.g., girls climbing trees) and friendship preferences than did younger children. Moreover, children reported they would react more negatively toward males than toward females exhibiting cross-gender friend and toy preferences. While younger children were unable to generate sex-typed traits reliably older children were and reported that they would respond negatively toward children with cross-sex-typed traits. Finally, although the vast majority of children at all ages indicated that cross-gender behavior in their peers was not wrong, they also indicated that they would prefer not to associate with children who violated sex-role norms. The results supported the notion that children are strict enforcers of sex-role norms and suggest that children's beliefs about their peers' reactions toward cross-gender behavior may limit the expression of sex-inappropriate behavior during the elementary-school years.