This study was an investigation of how mathematics teachers implement Strengthening of Mathematics and Science in Secondary Education (SMASSE) principles of Activity, Student-centred, Experiment, Improvisation (ASEI) through the Plan, Do, See, Improve (PDSI) approach. Social constructivism and PDSI were the main concepts that guided the research design. The study generated qualitative data through interviews, lesson observations and document analysis. One interview was conducted at the beginning of the study. Soon after its teachers worked together in a Community of Practice (CoP) for one school year and a second interview was administered at the end of the year. Guided by the PDSI conceptual framework, data were analysed and clustered around the themes of Planning, Implementation, Reflection, Improvement and knowledge of ASEI/PDSI. The following questions guided the study: How do teachers plan and deliver ASEI/PDSI lessons in mathematics? How do teachers reflect and improve on their lessons in mathematics? What challenges do teachers encounter in implementing ASEI/PDSI lessons and how can they be addressed? How does CoP support teachers in implementing ASEI/PDSI in mathematics? Its sample comprised 6 mathematics teachers from 3 secondary schools in South East Education Division of Malawi, purposively selected to target those that had been trained by SMASSE INSET Malawi. The study found that teachers had limited knowledge of some of the SMASSE principles of ASEI/PDSI due to poor accessibility of information and lack of support from each other. However, when teachers worked together in the CoP, their understanding and practice of ASEI/PDSI improved. The study, therefore, recommends that teachers work together in a CoP.