This study explores informal health education with a moralistic content in three Kenyan teacher training colleges and what it means for the development of a professional identity in health education student-teachers on a continent affected by far the largest number of health problems. Informal health education with a moralistic content is a kind of non-curricular health education which exists parallel to formal health education lessons, but which influences student-teachers' professional identity formation in complex ways by provoking resistance but also strengthening the community of student-teachers. The study used ethnographic methods and drew on a body of interrelated works in the field of sociocultural and critical educational theory and theory about professionalism to understand informal health education learning and processes of acquisition of professional identity. The findings document that in spite of institutional discipline and student-teachers' resistance to informal moralistic health education, informal health education also initiates peer learning and identity work as student-teachers negotiate what they consider an appropriate teacher identity in the complex structures of teacher training colleges. The study concludes that these processes strengthen student-teachers' sense of belonging to the teaching profession and thereby positively influence their professional identity.