Gareth Morris’s innovative research on the physiology of obligate anaerobic bacteria has greatly advanced our knowledge of fundamental mechanisms of life in the absence of air. His discovery of the multiple bases of aerointolerance in bacteria has exposed the inadequacy of previous theories which sought to explain obligate anaerobiosis in terms of deprivation of a single protective agent and has led to the formulation of an equilibrium view of oxygen tolerance that has gained general acceptance. Gareth’s important work on the growth and sporulation of saccharolytic clostridia has concerned particularly: effects of alien electron acceptors on fermentation mechanisms; the transport of fermentable substrates; and the properties and regulation of the membrane H+-ATPase. These studies contributed to the growing interest in biotechnological exploitation of anaerobes.