Many researchers believe that prior knowledge is the single most important variable influencing learning. Learning in the classroom is determined or affected positively or negatively, to some degree, by the knowledge people bring to the classroom. This study was designed to: (1) assess fifth- and eighth-grade Korean students' misconceptions of concepts related to photosynthesis; and (2) investigate change in students' concept understanding as it related to logical reasoning ability, textbook emphasis and background variables. Results showed that students in both groups had misconceptions of concepts related to photosynthesis--the meaning of making food, the use of light by plants, the functions of plant roots and leaves, photosynthetic products, and requirements of elements. Reductions in misconceptions between grade 5 and grade 8 was on items regarding the use of light by plants, exchange of materials between organisms in a system, knowledge of glucose, protein, fat, and nitrogen, and the skill of reading a graph. Appendices include a concept map of photosynthesis, tests, the questionnaire used, variable labels, correlation matrices, and a comparative achievement analysis. (RT)