This study was designed to examine the effects of adults' labeling and use of social groups on preschool children's intergroup attitudes. Children (N=87, aged 3–5) attending day care were given measures of classification skill and self-esteem and assigned to membership in a novel (“red” or “blue”) social group. In experimental classrooms, teachers used the color groups to label children and organize the classroom. In control classrooms, teachers ignored the color groups. After 3 weeks, children completed multiple measures of intergroup attitudes. Results indicated that children in both types of classrooms developed ingroup-biased attitudes. As expected, children in experimental classrooms showed greater ingroup bias on some measures than children in control classrooms.