The present study investigated a teacher's response to her pupils' disruption of the structure of classroom lessons. In an elementary school second grade classroom (19 boys and 16 girls), arithmetic and Japanese lessons were observed for 44 hours, from May to December (the school year started in April). The teacher was in her forties, and had taught for approximately 20 years. From the observational records, classroom disruptions were analyzed quantitatively by categories and qualitatively by content. The categorical analysis showed that associative disruptions were presented by many children, whereas unrelated disruptions and rejections were presented by particular children. The teacher's response was related to the kind of disruption. Reprimands increased when children chatted and disrupted the structure of the class. She usually responded to unrelated disruptions and rejections, but rarely paid attention to associative disruptions. Qualitative analysis indicated that the teacher changed her way of responding based on the level of structure of the lesson, usage of the pupils' ideas, and management of relationships in the classroom. These results suggest that this teacher did not conduct teaching and classroom management separately, but rather shifted back and forth between those two.