This unit, intended for students in grades eight or nine, is a revised version of ED 053 990. The teacher's guide lists the aims of the unit, behavioral objectives, suitable references and audio-visual aids, required apparatus and materials, and provides teaching notes for each activity, including comments concerning microbiological techniques. The core activities introduce major ideas concerning micro-organisms, particularly their role as decomposers in ecological cycles and their importance in economic production (bread, fermentation products). Students also choose from the following options: "Hunting Micro-organisms" (detection of bacteria and fungi in air, soil, and on body surfaces); "Other Micro-organisms" (pond-water examination); "Keeping Food Fit to Eat;""What's Brewing?" (cider, vinegar, beer, wine); "Getting Rid of Garbage;""Unseen in the Soil" (more extensive soil examination); "Disease" (infection in apples and goldfish); and a library assignment on "A Short History of Microbiology." The student's book provides instructions for some basic procedures, suggests experimental investigations, poses questions that are to be answered in a record book provided, and, in some cases, provides comments on the expected answers in an appended section. (AL)