A microcosm is a small, completely sealed, self-sustaining ecosystem. Once a microcosm has been sealed in a transparent container, only light and some heat can enter and only excess heat can leave. This manual describes how to set up aquatic microcosms using glass jars and little or no collecting equipment. The activities can be tailored to suit the abilities of students at any level and give students opportunities to learn about many kinds of freshwater invertebrates, watch an ecosystem develop and change, learn about food chains and webs, and use mathematics to calculate population trends. Some simple experiments that can be conducted with microcosms are also included. (JRH)