The lasting effects of teacher professional development (PD) are seldom examined. We investigated whether 44 teachers and their Grade 5 and 6 primary classes continued working with tasks for mathematical reasoning and employing a rubric after the PD finished. Questionnaires for students and teachers were administered before the intervention, at the end of the intervention, and 5 months later. The results of the longitudinal quantitative analyses with supplementary qualitative interpretations indicated that the mathematical reasoning features of the PD showed more sustainable effects than the use of the rubric. Explorative findings suggest that this outcome may be related to the teachers' pedagogical content knowledge.