The purpose of this study is to investigate knowledge structures of pre-service biology teachers on the “organic” concept. The study included two different samples for the first and second stages of the study, respectively. In the first stage, the participants (n=108) completed a word association task and they produced 172 concepts. 22 of the concepts indicated in higher frequency than 10 were given to the second group participants (n=20). The second group participants selected and used 9 of 22 concepts to develop concept maps in which “organic” concept was placed on the center. In addition, they wrote their definitions of organic and gave examples for it. By focusing on the concept maps and their definitions; knowledge structures of the participants were determined based on types of the maps, main titles found in the maps, links represented as sentences, definitions and the examples. The findings showed that two different types of the maps were determined as branched and hierarchical, also five main titles (food, chemistry, biology, health and agriculture) were found. Moreover, their definitions and examples also provided similar titles (food, chemistry, biology, health, textile and agriculture) except for textile. The links between the concepts in the concept maps, definitions and examples regarding organic concept showed that pre-service biology teachers had compartmentalized knowledge structures regarding “organic” concept. In conclusion, on associations between different concepts regarding “organic” concept, problems in terms of coherency were also determined.