OUR knowledge of the metabolism of the bacteria of the genus Clostridium (the strict anaerobes) is at present very scanty. It has been recently reviewed [Topley and Wilson, 1929; McLeod, 1930; Stephenson, 1930], so only a brief summary will begiven here, includingfacts discovered since the above reviews were published. Special reference will be made to Cl. sporogenes, which has been used by many workers as a type species, and of which our knowledge is the fullest. The present position is as follows.(1) These bacteria cannot grow in media containing an appreciable concentration of free oxygen. The degree of tolerance to low concentrations of oxygen varies from species to species. With regard to the reason for this lack of tolerance of oxygenno final decision has been reached. McLeod and Gordon [see McLeod, 1930] claim that it is due to production of hydrogen peroxide, while Quastel and Stephenson [1926] believe that oxygen acts by preventing the culture from reaching a sufficiently low oxidation-reduction potential.(2) They are unable to grow except in media containing protein or aminoacids. Carbohydrates, if present in addition, are fermented, but are neither sufficient nor essential for growth.(3) A compound containing the-SH group, or one from which the bacteria can produce an-SH group, is essential for growth (Quastel and Stephenson [1926] for Cl. sporogenes, and Burrows [1933] for Cl. botulinum).(4) Tryptophan is an" essential" amino-acid for Cl. sporogenes [Fildes and Knight, 1933]. With other species opinions differ; for instance, with Cl. botu-linum Fildes and Knight say that tryptophan is" essential," Burrows [1933] that it is not. The word" essential" is here used in the special sense that the bacteria require it because theyare unable to synthesise it.(5) In the case of Cl. sporogenes growth depends also on the presence in the medium of a trace of a vitamin-like substance [Knight and Fildes, 1933].(6) Burrows [1933] showed that Cl. botulinum can grow on a synthetic medium containing no amino-acids except glycine, alanine, leucine, proline, lysine and cysteine.(7) Wolf and Harris [1917; 1918-19] analysed the products of the action of Cl. sporogenes on various natural media, but could deduce little as to the reactions by which the products had been formed.