This study aims to reveal pre-service science teachers' (PSTs) levels of knowledge about the human reproductive system and their professional competence perceptions. The study was conducted using the case method. A form consisting of 6 open-ended questions was developed and applied to PSTs. The sample was composed of 62 PSTs, who were 3rd and 4th-year students in the Science Teaching Department of a university. It was found that the majority of the participants had a superficial understanding of the human reproductive system and they had various alternative concepts about the subject. The students attributed their incomplete understanding of the subject to the fact that they did not learn the subject effectively because they did not make a regular revision of their lessons, because the subject was taught superficially, and they felt embarrassed while the subject was being taught to them. It can be said that social life and the way individuals look at the subject play an essential part in obtaining such results. Based on the results, it can be said that offering teachers training on the subject by experts can yield positive results in teaching the subject.