As part of an ongoing study of the content knowledge, instructional beliefs, and instructional practices of middle school, high school, and college science teachers, the hypothesis that there are systematic differences across academic levels in these teachers' conceptual understanding of the same content-specific subjects was studied. Eight middle school, 10 high school, and 9 college professors have participated to date. An instrument has been developed with input from teachers to obtain information about commonly held teacher beliefs about science instruction, classroom practices, and student learning. Twelve science concepts were identified, representing three domains of the life science curriculum. Taken together, results indicate what may be fundamental differences across academic levels in life science teachers' understandings of concept meaning and interrelationship. These differences appear to exist largely independently of college coursework and teaching experience, and to include content that is common to all examined levels of life science instruction. To the extent that systematic and significant differences in science teachers' knowledge structures exist across academic levels, science students in transition from one level to the next are likely to experience confusion and frustration. One table lists categories of participants, mean number of college courses completed in six subjects, and years of teaching experience. (Contains 13 references.) (SLD)