We describe part of an action-research programme in Spain which was based on metacognitive reflection. The participants were four science teachers in a secondary school during the 2004-05 and 2005-06 academic years. During the study, they each analysed their own pupils' alternative ideas on photosynthesis and their teaching methods as recorded in videos of their classes, and followed this by planning new teaching units. The present communication focuses on the case of one experienced teacher. The results showed that the teacher's reflection on his pupils' commonest alternative ideas and his own classroom teaching led him to plan new teaching units for the second year of the study which took those alternative ideas into account, and included new strategies, resources and activities. The programme has contributed to the teacher's professional development, impacting significantly on the elements that form part of his teaching, and positively affecting the learning and conceptual change of his pupils. (Contains 4 figures and 1 table.)