The levels of viable indigenous bacteriophages and human enteric viruses contained in raw water entering a full scale drinking water treatment facility were examined on a quarterly basis for a 1-year period. On these same sampling occasions, indigenous virus concentrations were also determined for water samples collected at the post sedimentation, post sand filtration, and finished (post chlorination) stages of treatment. Identically paired water samples were seeded with laboratory viruses to assess whether viral detection efficiency differed for water at various stages of the water treatment process. The water samples examined during the course of the study were also tested to determine their respective levels of several physical and bacterial indicators. Average values for the efficiency of viral detection were unaffected by progressive stages in treatment of water at the plant. The latter finding possibly suggests a role of suspended water particulates in the viral concentration process.