The suitability of certain species of protozoa as indicators of water quality has been determined. Experiments were conducted under laboratory conditions to standardize a bioassay procedure for water quality using eitherParamecium caudatum.Amoeba protpus,orEuglena gracilisas the indicator organism. The bioassay, which consists of exposing the organisms to a known concentration of pollutant under laboratory conditions, followed by microscopic observation to establish the time of death, affords a reliable, convenient and inexpensive way to monitor for water quality.