This study investigated how pre-service biology teachers perceived the concepts of 'system' -- as the basic component of the systems thinking approach -- and 'biological system' specific to biology. The most important characteristics of a system appear as elements, interactions, interconnections, hierarchy, purpose and forming a whole. The data of the study were collected from students who were studying at the Faculty of Education of a state university in Ankara. The study employed the basic qualitative research approach. A questionnaire consisting of open-ended questions and interviews was utilised to collect the data. The sample consisted of 150 pre-service teachers for the questionnaire and 15 pre-service teachers for the interviews. The collected data were subjected to thematic analysis. It was determined that the pre-service teachers had a general perception of a 'system' and a 'biological system', but this perception was not as strong as a perception that they could apply in their own educational processes. Systems thinking approach and system concept are critical not only for pre-service biology teachers but also for all other fields. It is considered that it is necessary to positively contribute to the perception of pre-service teachers who are the teachers of the future towards these concepts.