The Science Curriculum Improvement Study has developed this teacher's guide to "Populations," the third 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; and they make use of scientific and process-oriented concepts appropriate to the study of life. In "Populations," attention is directed toward the feeding relationships among populations of organisms. To understand such food relationships in nature, students construct aquaria and terraria to observe interacting populations. This serves as background for developing the concepts of population, community, food chain, food web, predator-prey, plant eater, and animal eater. Topics of the four-part unit are entitled: Populations, Food Relationships in the Terraria, Food Relationships in the Aquaria, and The Community. 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, procedures to follow, 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)