Direct predator-prey relationships involving a few species of ciliated and flagellated protozoans from the periphyton of oligotrophic waters appear to be the major mechanism in such environments responsible for removal of fecal bacteria. Other factors, such as the indigenous bacterial flora and the physicochemical properties of these waters, are relegated to a more indirect role. The lengthy lag period (4-5 days) required for initial predatory responses indicated the delicate nature of high-quality waters and the need for strict sanitary guidelines to ensure their future safety and quality. The sanitary quality of oligotrophic waters can best be determined by the enumeration of fecal streptococci in conjunction with fecal coliforms.