Derived from the perspective which views teachers and students as rational humans with limited capacity of perception and reasoning, this study describes how teachers differentially treat students and examines what underlies the differential treatment. Three science teachers from an urban middle school in the Midwest participated in the study. Classes were observed; formal and informal interviews with teachers, administrators, and students were held; and various kinds of instructional materials were gathered. Findings are summarized as they relate to treatment of whole classes, individual students, and teachers' perceptions of their role. Teachers' treatment of whole classes is described under the topics of content taught, teaching strategies, attitudes toward students, and evaluation of students' performance. Teachers' treatment of individual students is reported based on their actions in handling disruptive, passive, and highly motivated and able students. An overview is presented on how teachers differentially perceived their whole classes and individual students within a class. The possible reasons behind teachers' perceptions and actual treatments are then discussed. A reference list is also included. (ML)