The writer discusses the evidence pertaining to the theories of sulphate reduction in oil-well waters. First, the theory that sulphates are reduced by inanimate organic matter is reviewed in the light of field and laboratory evidence. Studies of the earth’s sediments do not indicate that the temperatures have been sufficiently high to bring about the reaction. Second, the biologic concept of sulphate reduction is reviewed in the light of recent field and laboratory investigations. The latter theory is supported by considerable fact evidence. There is some discussion pertaining to ecologie questions in relation to the life processes of sulphate-reducing micro-organisms. The significance of this biologic method of sulphate reduction has not been established. Part of the literature is cited.