Background: Formative assessment, as an integral component of teaching, has recently gained prominence in educational environments globally. Poor performances in mathematics by learners in early grades, and its negative effect on later learning, have been an ongoing concern in South African schools. Several former studies tend to generalise the pedagogical reasons for learners' underperformance in Foundation Phase teaching. Aim: This case study of selected Grade 3 teachers examined how the teachers integrated formative assessment into their pedagogy, with the purpose of gaining insight into teachers' understanding of the developmental aspects of learning in mathematics. Setting: This study was conducted at four schools in a selected district in the Gauteng Province. Methods: Data were mainly collected through focus group interviews and observations of at least three classroom sessions for each teacher of mathematics, thereby gaining an overview of their formative assessment practices. Results: This article reports on the two strongest themes to have emerged from the case study, which were teachers' tokenistic use of 'Assessment for Learning' strategies and teachers' awareness of learning processes and curriculum requirements. Conclusion: The study's main conclusion was that teachers are likely to practise formative assessment more intuitively if they had a sound knowledge of children's mathematical cognition and conceptual development. This study pointed out that formative assessment is a co-constructed activity involving the teacher, the learner and peers rather than a teacher-directed activity. The study recommends how continuous professional learning initiatives can design initiatives that integrate research-based knowledge of children's learning of early grades mathematics.