The teacher ratings were conducted with the use of the Social Competence and Behavior Evaluation (SCBE), which is a questionnaire that examines social competence and behavioral/emotional adjustment in children ages 30-78 months by tapping overall emotional expression, social interactions with peers, and interactions with teachers. SCBE data were collected for 2 consecutive years at each Head Start program. The prevalence of externalizing and social competence problems in this sample of preschool children was significantly higher than expected in this community of low-income African-American families, and there was wide variation in prevalence across preschool sites and classrooms. The incidence of significant problems in several behavior categories was negligible in certain classrooms but over 40 percent in others. The most marked variation occurred in ratings of aggressive behaviors. The wide variation in ratings across classrooms has several possible explanations, including teachers' use of different standards in their ratings, differing levels of teacher experience and/or burnout, and varying rates of pre-existing behavioral problems in children. Although differences among classrooms outweighed differences in teacher education/training, teachers with the same level of formal training may differ in their abilities to develop organized, prosocial environments in the classroom. Future research should focus on how classroom structure and management practices are linked to the children's behavior and the effectiveness of various interventions. 1 figure, 3 tables, and 28 references