The use and analysis of student generated propositions linking two scientific concepts (photosynthesis and respiration) were examined before and after the study of photosynthesis. A proposition-generating task (PGT) was used as a diagnostic tool to indicate student misconceptions and partial understandings. Focus was on determining: how the nature of the link between photosynthesis and respiration changes following instruction, and teachers' attitudes toward the PGT. A sample of 11th-grade and 12th-grade biology students participated in an evaluation of a curriculum for teaching photosynthesis and related concepts. One group of subjects (n=516) studied photosynthesis using the experimental materials, while a comparison group (n=147) studied photosynthesis using regular course materials. Students were asked to write propositions showing the relationship between photosynthesis and respiration before and after instruction. Propositions were grouped into categories reflecting understanding of the concept. Instruction with the experimental materials resulted in a decrease in major misconceptions, an increase in the frequency of goal concepts, and an increase in evidence of student understanding. The task was also well-received by teachers as a diagnostic measure. Four tables, one figure, and an 18-item list of references are provided. An appendix contains examples of student-generated propositions. (SLD)