Microbes, through their specialized metabolic processes and ecologies, play a fundamental part in man's economy, and their economic roles are widely recognized as important. The sulphur bacteria have had in past geological eras, and are having today, numerous and diverse effects, often deleterious, scattered throughout the economy. Many of their economic roles are not widely known. Their economic and environmental effects arise, according to the type or combination of species involved, principally from (1) the production of acid, (2) the formation or removal of hydrogen sulphide, and (3) the removal of oxygen. In this paper their various effects on industry and the environment are surveyed.