This comparative study analyzed the similarities and differences in the mathematical communication used by teachers and primary school students in Japanese, Lao, and Thai mathematics classrooms. It adopted a qualitative research design, and the targets were teachers and students in Grades 1-6 at a selected school in each of the three countries. The data were collected from one classroom at each grade level from the three schools, so eighteen classrooms were involved in the study. The research tools included video recorders, photo cameras, and a field notes form. The data were analyzed by employing the analytical descriptive method based on Pirie's conceptual framework of the means of mathematical communication (1998). The findings revealed that the means of mathematical communication in the three countries were similar in their use of ordinary, mathematical, verbal, and symbolic language--differences in the means of mathematical communication related to how varied the learning materials were. The Japanese classrooms used the most varied means of mathematical communication, demonstrating all six means, while the Thai classrooms used only 5, and the Lao PDR classrooms used only 4. The Japanese classrooms were shown to be focused on the students' diverse self-solution concepts, while some of the Thai and Lao PDR classrooms were based on question-answer interactions between the teachers and students.