If classrooms are dynamic, then mathematics teachers will need to improvise mid-lesson from time to time. Teachers' capacity to improvise is usually analysed via a cognitivist lens. This study contrasts a cognitivist analysis of a primary teacher of mathematics with an ecological analysis. The ecological approach was able to develop a more detailed characterisation of teacher improvisation where attention to, and manipulation of, environmental entities supported improvisation. This characterisation of improvisation is posited to have potential in developing novice teachers' capacity to 'think on their feet' in mathematics lessons.