This study has aimed at revealing the knowledge for teaching a middle-school mathematics teacher has in teaching the 5th-grade subject of fractions. For this purpose, the Mathematics Knowledge for Teaching (MKT) was used. The study adopted the holistic single-case study, one of the qualitative study designs. The study was implemented with a teacher assigned at a public school and who volunteered for the study. The study data were collected by semi-structured interviews held with the teacher and observations during the teaching process of the subject of fractions, on which the teacher’s knowledge was sought to be assessed. Consequent to the study, it was revealed that the middle-school mathematics teacher possesses insufficient content knowledge on fractions, operations with fractions and meanings and models of fractions. It was concluded that his insufficient content knowledge also had an adverse impact on this knowledge for teaching and therefore, restricted the teacher’s teaching process. Based on the study, it was concluded that due to the teacher’s limited content knowledge and pedagogical content knowledge, he has an insufficient mathematical knowledge for teaching.