The F-Sort of Biology Concepts was used to assess understanding of the relationships among 37 biology concepts by five groups: Preservice secondary science teachers, in-service biology teachers with 1–3 years of teaching experience, in-service biology teachers with 5 or more years of experience, scientists in any biological science field, and college seniors majoring in biology. Data collected from the F-sort were analyzed using latent partition analysis and alpha factor analysis with additional interpretation from multidimensional scaling. The subjects were asked to think aloud as they performed the F-sort and each session was audiotaped for later analysis. These analyses indicated that the biology major and experienced secondary science teachers were separated from the scientists by a dimension based on a deep-versus-surface structure understanding of the concepts. A second axis shows that scientists are separated from other groups by a fluid-versus-fixed cognitive structure dimension. That is, both experienced teachers and scientists were found to have well-constructed and ordered cognitive structures, but scientists were much more likely to see an item having a place in two or more categories, whereas experienced teachers tended to focus on only one aspect of an item, and therefore understanding that it rightfully belonged in only one category. It appears that teachers restructure their science knowledge as they become more experienced. There is an apparent transition from poorly organized to highly organized cognitive structures for biology concepts when comparing preservice, novice, and experienced teachers, respectively. The transition does not seem to be one achieving a deeper understanding of the biology concepts or to a greater degree of integration of the concepts, but rather a transition from a fairly large, loosely organized pool of biology concepts to one which is highly structured but limited to the expectations of the established curriculum. The results have implications for the well-known conjecture that teaching helps one better understand the content being taught.