Abstract f2 Phage, attenuated Polio I (LSC) strain introduced daily to a 350 l. experimental oxidation pond showed no decrease in bacterial viruses f2 or other coliphages or Polio I strain. Ratios of coliphages to human enteric viruses ranged in flood waters from concentrations as low as 1:1 to as high as 10 3:1; in wastewater at various seasons the ratio was 10 5:1; in trickling filter effluents in winter it was 10 4:1; in spring 10 5:1, in summer and fall 10 4:1, in oxidation pond effluents in winter 10 3:1; in spring 10 4:1; and in summer and fall 10 3:1. Out of three epidemics in small communities caused by failure of water supply, coliphages were found to be positive. At the same time only two samples of human enteric viruses were positive (the third was contaminated with yeasts). Chlorination experiments using the experimental oxidation pond showed that f2 was most resistant, MS 2 was very resistant, and coliphages were more resistant than attenuated Polio I virus. Experiments with the oxidation pond effluents showed that coliphages were at least as or even more resistant to chlorine than human enteric viruses.