This study aims to improve the pronunciation of -ed endings of regular verbs among 35 ninth-graders through the implementation of lesson study at a public school in the city of Loja, Ecuador during the 2021-2022 school year. This study employed a mixed method with a quasi-experimental design. Data were collected through a pretest and posttest, a qudestionnaire, and field notes which were registered by the researcher as a participant observer and three collaborators as non-participant observers. After an eight-week period of time, results demonstrated that lesson study helped ninth-graders to improve their pronunciation knowledge related to -ed ending sounds (/d/, /t/, /id/) as they moved from ineffective or partially effective pronunciation to good pronunciation.      Additionally, among the four strategies used for pronunciation instruction, students expressed that “songs” were the most effective for their pronunciation improvement. Furthermore, it was beneficial for the researcher as it contributed to his openness to reflect upon his collaborators’ observations, which helped him to develop more effective lesson plans.