The current study aims to determine science student teachers' cognitive structure on the concept of food chain. Qualitative research method was applied in this study. Fallacies detected in the pre-service teachers' conceptual structures are believed to result in students' developing misconceptions in their future classes and will adversely affect their future teaching performance. The data were collected from 48 science student teachers. A free word association test was used as a data collection instrument. The data collected were subject to content analysis. Analyzing the science student teachers' responses to the concept of food chain on the free word association test, these responses were coded and divided into categories. Based on the categories, frequency and percentages were provided. The data collected through the study were divided into 8 categories, which were stated as follows: energy flow--producer, consumer, decomposer organisms- food chain and food pyramid relationship--ecologic factors--matters used in energy flow--terms related to vitality-deterioration in food chain--the characteristics of the population. When the words provided as answers by the science student teachers to the concept of food chain were analyzed, it was noticed that they had more word connections with energy flow and in terms of the elements that compose food chain. Moreover, it was determined that they had some misconceptions about food chain. Similar research can be conducted with different student groups and for the correction of alternative concepts related to the concept of food chain, Extra biology courses should be included in undergraduate curriculums.