The Science Curriculum Improvement Study has developed this teacher's guide to "Ecosystems," the sixth part of a six unit life science curriculum sequence. The six basic units, emphasizing organism-environment interactions, are organisms, life cycles, populations, environments, communities, and ecosystems. They make use of scientific and process-oriented concepts appropriate to the study of life. Ecosystems develops the overall concept of an ecosystem, a community of organisms interacting with its environment. It stresses, in addition, the concepts of pollutant, water cycle, oxygen-carbon dioxide cycle, and food-mineral cycle. This unit serves as a culmination of the five preceding units and exemplifies the program's unifying theme - the ecosystem. Topics of the five-part unit are entitled: Classroom Ecosystems, The Water Cycle, The Oxygen-Carbon Dioxide Cycle, Cycles in an Ecosystem, and Pollution. Suggestions are presented for the teacher to use an exploration-invention-discovery approach to learning together with points to consider regarding teaching materials, advance preparation, and optional activities. Activities and experiments lead to clarification of the concepts and provide experience in measurement skills and critical thinking by the students. (BL)