This study aimed at exploring whether and how learners’ tasks planned by teachers and their implementation promoted learner participation in rich mathematical talk during lesson study on Pythagoras Theorem. Data included two lesson plans and two lessons planned and delivered by a group of five teachers from one secondary school in Malawi. Each lesson was video recorded, transcribed and divided into lesson episodes for content analysis using codes developed from the Mathematics Teaching Framework. The findings indicated that the teachers were able to develop tasks which could support learners’ participation in rich mathematical talk. However, realisation of such rich mathematical talk for learners was a challenge during task implementation. I conclude that promotion of rich mathematical talk among learners is difficult and cannot be achieved through one lesson study cycle. This calls for the need for regular continuous professional development to promote in-service teachers’ capacity to engage learners in rich mathematical talk as well as enriching pre-service mathematics education content with such opportunities.