The aim of the current study is to investigate student biology teachers' cognitive structures related to "diffusion" through the free word-association test and the drawing-writing technique. As the research design of the study, the qualitative research method was applied. The data were collected from 44 student biology teachers. The free word-association test and the drawing-writing technique were used as data collection instruments. The data were subject to content analysis and divided into categories through coding. In the analysis, the categories were formed and determined through the results of word-association test and drawing-writing test which were completed by the student biology teachers. With the help of these categories, the cognitive structures of student biology teachers were explained. The data collected through the study were divided into 8 categories ("defining diffusion," "diffusion environments," "diffusion-forms of substance transfer," "places and structures where diffusion occurs," "substances in diffusion and their characteristics," "importance of diffusion in plants," "examples of diffusion" and "factors influencing diffusion speed"). The common and dominant category in both assessment instruments is "defining diffusion". On the other hand, it was determined that student biology teachers had alternative concepts related to diffusion. At the end of the article, comprehensive suggestions are presented on the subject.