The aims of this study were to diagnose students' misconceptions concerning photosynthesis and respiration in plants, and to investigate reasons behind these misconceptions. The subjects were 45 ninth grade high school students and 11 high school teachers. Data were collected by interview technique. All of the interviews were audiotaped and transcribed verbatim by the researcher. Misconceptions were analyzed under the headings of plant nutrition, role of water in plants, role of leaves in plants, gas exchange, energy transformation, the definition and the importance of photosynthesis, role of light in photosynthesis, respiration in plants, and the relationship between photosynthesis and respiration in plants. Findings indicated that many students have misconceptions about photosynthesis and respiration in plants. These misconceptions were found to be generally based on social practices and school experiences involving students' learning styles and instruction. (Contains 15 references.) (Author/YDS)