SUMMARY: The selenate ion, a remarkably powerful competitive antagonist of the reduction of sulphate by Desulphovibrio desulphuricans (Hildenborough) suspensions, did not affect reduction of sulphite and thiosulphate. Cysteine, methionine and glutathione had no anti-selenate effect. Selenate also inhibited the growth of D. desulphuricans ; this effect was antagonized competitively by sulphate and non- competitively by sulphite. Repeated subculture in subinhibitory selenate + sulphate mixtures did not give rise to a selenate-resistant strain, though selenium was deposited in these conditions. The monofluorophosphate ion behaved similarly : it was a competitive sulphate antagonist in growth and in sulphate reduction, though it had a lower specific anti- sulphate activity than selenate. It did not affect the metabolism of sulphite or thiosulphate. At high concentrations it showed non-competitive inhibition of sulphate reduction. Of the other 'analogues' of sulphate tested, potassium tellurate suspensions and chromate inhibited growth and sulphate reduction, but were not competitive sulphate antagonists. High concentrations of perchlorate depressed sulphate reduction in a non-competitive manner, and methanesulphonate, P-hydroxyethanesulphonate, benzenesulphonate, ethylsulphate, sulphamate and dimethylsulphone were inactive. An acridinium dye known to inhibit the growth of D. desuZphuricans did not affect sulphate reduction, and, in growth, was not antagonized by sulphate or complex nitrogenous supplements. The organism was readily ' trained' to resist this dye. The control of the growth and metabolic activities of Desulphovibrio desul- phuricans is a problem of practical as well as of theoretical interest, in view of their role both in the anaerobic corrosion of ferrous pipes (von Wolzogen Kuhr & van der Vlugt, 1934; Bunker, 1936,1939; Hadley, 1943; Rogers, 1940, 1945; Starkey &Wight, 1945; Starkey, 1947; Butlin &Vernon, 1949; Chemistry Research, 193846,1947, 1948, 1949) and in the pollution of water and sewage (Bunker, 1942; Rogers, 1945; Allen, 1949; Butlin, 1949). Hence the inhibition of these bacteria has been studied largely from a practical standpoint. Rogers (1940) investigated the resistance of crude cultures to various dyestuffs and observed that acriflavine and proflavine were their most active inhibitors. He studied in detail a dye of the acridinium group, '914', and showed that its activity depended on inoculum size. He noted the possibility that adaptation to resist the dyestuff could take place, and a further publication (Rogers, 1945) described the practical use of the dye. Bunker (1942) advocated decrease of pH value to control the pollution of relatively small water volumes by sulphate reducers. Allen (1949) confirmed this in a study of their activities in sewage, mentioned their high resistance to chlorine and showed that various nitro-compounds depressed H,S formation. The resistance of these organisms