The research reported in this paper concerns the design of instructional materials that represent the content structure of a science discipline and the development of methods of probing and representing the knowledge structure in a student's memory. The science discipline selected for the study was geology. Specifically, the conceptual structures and related taxonomy for classifying rocks were used in the instructional materials designed for the study, which consist of a segment of the field-testing version of the Individualized Science (IS) program. Thirty students from an eighth-grade class in a parochial elementary school were selected for the study. None of these students had previously received instruction on minerals and rocks. All had previously studied IS units, however, and were thus familiar with the mechanics of the instructional materials. A preinstructional concept structuring task was administered to probe for students' knowledge about rocks and minerals. A pretest on concepts in descriptive geology was then administered. Instruction followed. Posttest and postinstructional concept structuring task administration occurred after instruction. Test results were analyzed in detail. Implications for the administration of probing tasks are discussed, particulary for applicability in instructional techniques and determination of content difficulty for particular students. (CS)