This study examines the pre-service teachers' opinions about conceptual integration (CI) and their understanding of it. A qualitative phenomenology design was used in the study. Data was collected through in-depth semi-structured interviews comprising ten guiding questions. Three pre-service physics and three pre-service chemistry teachers participated conveniently in the study. While the pre-service chemistry teachers took a course regarding the CI explicitly taught, the pre-service physics teachers were not subjected to any information relating to the CI in their course. Data was analyzed by forming the codes and themes. Generally speaking, while the pre-service chemistry teachers think that physics concepts should be used in the chemistry lessons, the pre-service physics teachers believe that these two subjects' concepts generally are not related to each other. Moreover, the participants had some difficulties in understanding the CI between physics and chemistry concepts. The possible implication for science teacher education and research is discussed.