It has been shown that low concentrations (1,000 p.p.b.) of ortho and para-chlorophenol, when added to the ordinary dilutions of domestic sewage, were not always removed in periods of 20 to 30 days at 20C. Similar concentrations of these mono-chlorophenols were removed at this temperature during similar periods of storage when added to polluted surface waters. These observations on the conditions required for the dissipation of mono-chlorophenols in surface waters and sewage dilutions along with previous observations on the similar dissipation of cresols and phenol indicate that biochemical dissimilation is responsible for the removal of these materials. The study has also shown that the removal of these mono-chlorophenols requires the presence of a specialized microflora.