This case on conceptual change teaching aims to find out the influence of cooperative learning on students' understanding of some selected concepts in high school chemistry. It was done in actual classroom setting within a four-month duration using a sample of 30 third year students from two intact classes of a science high school taught by the same chemistry teacher. The study examined the nature of the conceptual changes that took place among the subjects and attempted to identify the emerging conditions that seemed to encourage or hinder the process of conceptual change. Data were gathered through classroom observations, written and oral interviews and questionnaires. Concepts related to structure of matter and chemical bonding were selected for investigation. Phenomenographic analysis of the students' answers in the explanation portion of the pretest and posttest generated a set of categories of description for each concept. Conceptual trace analysis provided a classification of each of the students' responses to the multiple-choice questions. From the conceptual trace analysis results, the changes that occurred from the pretest to the posttest were determined. The patterns of interaction during cooperative learning were generated from the audiotaped discussion of students within the small groups.