The research explores the metacognitive behaviours demonstrated by secondary school students with different mathematical achievement levels (outstanding, high, moderate and low levels of achievement) during solving a mathematical problem. The research uses a case study method and involves the participation of 36 students. The data, which were collected with the multi-method interview technique, were analysed content analysis. Considering the analyses, it was found that the students with outstanding achievement levels demonstrated more behaviours in number, by spending more time during the problemsolving process; on the other hand, students with low achievement levels demonstrated less behaviour in number by spending less time compared with the other groups. It was found that the students with outstanding, high and moderate levels of achievement demonstrated evaluation behaviours most; in contrast, students with low achievement levels had the behaviours of awareness the most during the problem-solving process. In addition, it was concluded that the groups started the process generally with a behaviour in the dimension of awareness, used the behaviours of regulation and finished the process with behaviour in the dimension of evaluation. The teacher should guide students by focusing on some behaviours respectively related to the dimensions of awareness, regulation and evaluation during solving a problem in the course.