The occurrence of viruses in conventionally treated drinking water derived from a heavily polluted source was evaluated by collecting and analyzing 38 large volume (65 to 756 liter) samples of water from a 9m3/sec (205 mgd) water treatment plant. Samples of raw, clarified, filtered and chlorinated finished water were concentrated using the filter adsorption/elution technique. Nineteen of 23 (83%) samples of finished water contained viruses. None of the nine finished water samples collected during the dry season contained detectable total coliform bacteria, however, fecal streptococci were detected in 6 of 6 samples tested. Seven of 9 finished water samples collected during the dry season met turbidity, total coliform bacteria and total residual chlorine standards. Of these, 4 contained virus. The percent removals of rotavirus, enterovirus and coliphage were calculated to be 93.5, 25 and 100%, respectively. None of the 14 finished water samples collected during the rainy season met turbidity standards and all contained rotavirus or enterovirus.