This Teacher's Guide is designed for use with the Science Curriculum Improvement Study's (SCIS) unit Population. Populations is the third of a six-unit sequence of SCIS's Life Science Program for grades K-6. The Populations guide consists of activity outlines along with suggestions for guiding children's observation and manipulations of living systems, so that they can develop some concepts that are important for an understanding of an ecosystem. The activities are grouped into four major parts: (1) Aquatic Populations, (2) Terrestrial Populations, (3) Food Relationships, and (4) The Community. Heading each part is a list of objectives, followed by background information about the concepts and an overview of the chapters in that part. Each chapter begins with a brief synopsis of the children's activities, lists the materials to be used, and outlines necessary preparation techniques. The teaching suggestions describe activities that provide opportunities to help the children achieve the objectives. Each chapter contains information and instructions for the care of organisms. In addition, the teacher is sometimes referred to an appendix page which contains pictures and general information about the plants and animals used in the unit. (BR)