In order to acquire mathematical literacy and to develop mathematical proficiency, it is significant both to understand and to reflect on mathematical thinking. Because of the reflexive relationship between pedagogy and interaction (Seedhouse 2004), we analyze Finnish mathematics lessons to investigate how teachers and pupils communicate on mathematical tasks. Our specific aim is to analyze teachers’ reactions on pupils’ wrong answers. Our data consists of 16 videotaped lessons by different teachers in the 6th grade of comprehensive schools in Helsinki area. Most of the lessons concerned fractions and the main activity was to calculate and check answers for tasks in the textbooks. We use ethnomethodological conversation analysis to investigate the structure of the interaction (Schegloff 2006). We are interested in wrong answers because their reception carries a strong ‘pedagogical message’. In our data, there is a wide variety of teacher responses to the answers that are treated as ‘wrong’. We expect to offer new knowledge about the consequences of teachers’ different kinds of responses for classroom interaction and student participation. Since we see these interactional sequences as opportunities for learning and understanding mathematics (Melander & Sahlström 2011), we hope to gain new knowledge for the development of teacher training programs.