We present a laboratory-based exercise that is used to teach basic lab skills (e.g., aseptic technique and enumeration) using naturally occurring microbial communities in a real biological context. Students examine the colonization by microbial communities of leaves that fall into streams. Leaf decomposition reflects enzymatic activity by microorganisms such as aquatic fungi and bacteria and maceration by invertebrate shredders. The microorganisms help facilitate the cycling of nutrients and energy in the stream's ecosystem. This exercise effectively teaches students to use lab skills to quantify microorganisms found in nature, investigates groups of microorganisms involved in leaf degradation in streams, and stimulates interest in both microbiology and ecology.