The study examined two primary teachers' professional learning and joint knowledge construction in the context of co-teaching. The teachers narrated their learning as a collaborative process with serendipitous origins. Shared knowledge construction was crucial in the learning process, as was implementing the resulting new ideas in practice. It is concluded that experiences of co-teaching may support teachers in meeting their professional responsibilities effectively. Professional development programmes need to be sensitive to teachers' individual and collaborative learning experiences to be able better to support them in the natural context of those experiences in particular local and national contexts.