The study aimed to determine the noticing skills of mathematics teachers to their students' misconceptions in written responses. The study adopted case study design, one of the qualitative research methods. The participants of the study comprised 20 volunteer middle school mathematics teachers from 9 different cities and 16 different schools that were selected via convenience sampling method in 2017-2018 school year in Turkey. The schools were determined based on the examination called 'Nationwide Standardized Level Determination Exam' as high, medium and low achievement school levels. Considering the idea that teaching experience could affect noticing, special attention was paid while choosing teachers with different years of teaching experience. The data were collected using a "diagnostic test of misconceptions" developed by the researchers, and video-recorded clinical interviews with teachers. The diagnostic test included 13 open-ended questions, and was administered to 497 students (7th grades) by their own teachers. During the data analysis, first, the students' answers were examined in the context of misconceptions in order to identify the students who had misconceptions in their written responses. Then the participant teachers were interviewed considering their students' answers containing misconceptions. Finally, both types of data were analyzed via content analysis method. The findings showed that there were many students, who had misconceptions in their written responses, and the teachers' viewpoints regarding the misconceptions were narrow and their noticing skills to students' misconceptions were low. [This study was presented as an oral presentation at the International Conference on Mathematics and Mathematics Education, Ordu University, Ordu, Turkey, 27-29 June 2018.]