Student variables which influence attainment of ecological concepts and conceptual systems were investigated. Researcher-designed paper/pencil and free-sort categorization tests were administered to 256 Science Curriculum Improvement Study (SCIS) students in grades 4-6. Findings are: (1) grade and ability, not sex, influenced concept attainment and conceptual system; (2) sequence of concept development was not influenced by grade, ability, or sex; (3) sixth-grade students conceptualized life requirements, feeding relationships, food-mineral cycle, and community-environment interactions but not the higher-order classificatory concepts or oxygen-carbon dioxide cycle; (4) discipline experts, not elementary students, possessed conceptual systems based upon higher-order concepts; and (5) concept attainment was promoted by a conceptual system similar to the discipline structure. Recommendations include continuing concrete activities, investigating effectiveness of SCIS and other elementary programs in developing higher-order classificatory concepts and the oxygen-carbon dioxide cycle, emphasizing development of conceptual systems, and investigating teachers' ability to effectively utilize concrete experiences and concept maps. (Author/JN)