Using lesson observations, the study reported in this article explores how two practising secondary mathematics teachers implemented formative assessment actions in their classroom teaching. The study also investigated whether teachers' beliefs about teaching and assessment could be mapped onto their classroom practices. In particular, while the two teachers were implementing student portfolio assessment in their own Year 9 mathematics lessons, the classroom observations focused on how they utilised formative assessment actions such as clarifying and sharing learning criteria intentions and criteria for success; activating students as instructional resources; and, providing feedback that moves learners forward. The findings suggest that one of the teachers made better use of formative assessment or assessment for learning actions while the other showed an emerging understanding of such ideas. A holistic analysis of teachers' actions point to possible links to their beliefs about teaching and assessment. These findings imply that some teachers may hold productive beliefs about teaching and assessment that support the use formative assessment actions more readily. Such productive beliefs provide a useful platform for enacting better assessments inside secondary classrooms, given that there is paucity of research that deals with how secondary teachers make use of formative assessments. The findings implicitly confirm that better use of formative assessment strategies tend to result in more interactive lessons.