The aim of this case study is to define reflection activities of a lesson study group that consisted of three middle school mathematics teachers in Usak, Turkey. In the process, the participants worked in collaboration for about 5 months to implement three study lesson practices. The data were collected through observations in the reflection processes of the study lessons. Teachers' reflection activities were analyzed based on the literature of teacher self-regulation. As a result, lesson study encouraged teachers to make detailed and reliable evaluations, controllable causal attributions, and productive inferences for their teaching.