Background: It is important that mathematics teachers have a robust knowledge of the mathematics they teach. They need more than just knowledge of the content because they are expected to facilitate understanding of the content with their learners. This study focused on the knowledge of practising Grade R teachers from Lesotho. Aim: The purpose of this exploratory study was to explore practising Grade R teachers' mathematical knowledge for the teaching of numeracy. Setting: The study was conducted with 48 practising Grade R teachers while they were enrolled in an in-service programme at a College of Education in Lesotho. Methods: Data was generated through the use of questionnaires. The questionnaire included items focusing on the four domains of knowledge, namely common content knowledge, special content knowledge, knowledge of content and students, and knowledge of content and teaching. Results: The findings revealed that some teachers were unable to carry out division problems accurately and many struggled to explain the possible modification of a teaching plan. Only 27% of the group were able to explain that there were levels of understanding numeracy. With respect to ordering a sequence of three topics, only 15% of the teachers produced a reasonable sequence. Conclusion: These teachers' difficulties raise concerns about the effectiveness of their teaching of numeracy. Despite the Lesotho government's commitment to improving the learning of mathematics at the Grade R level, much more work is required to be conducted with teachers so that their mathematical knowledge for teaching (MKT) numeracy can be improved.