This study investigated the professional identity development of teacher candidates participating in an informal afterschool science internship in a formal science teacher preparation programme. We used a qualitative research methodology. Data were collected from the teacher candidates, their informal internship mentors, and the researchers. The data were analysed through an identity development theoretical framework, informed by participants' mental models of science teaching and learning. We learned that the experience in an afterschool informal internship encouraged the teacher candidates to see themselves, and to be seen by others, as enacting key recommendations by science education standards documents, including exhibiting: positive attitudes, sensitivity to diversity, and increasing confidence in facilitating hands-on science participation, inquiry, and collaborative work. Our study provided evidence that the infusion of an informal science education internship in a formal science teacher education programme influenced positively participants' professional identity development as science teachers. (Contains 2 notes, 3 tables, and 8 figures.)