Abstract: IN 1918 H. H. Green1 isolated from arsenical cattle-dipping fluids in South Africa a bacterium, provisionally referred to as Bacillus arsenoxydans but since lost2, which was able to grow in culture media cantaining up to 1 per cent of As2O3 as arsenite and to bring about its oxidation to arsenate. In view of the well-known toxicity of tervalent arsenicals, now largely explicable in terms of their inhibitory action upon highly important sulphydryl-dependent metabolic enzymes3'4, it is particularly unfortunate that Green's valuable paper, being published in an annual report mainly devoted to veterinary research, apparently escaped the attention of enzymologists and general bacteriologists ; for the enzymic mechanism of arsenite oxidation was never studied while the strain was available, nor was the existence of arsenite-oxidizing bacteria ever afterwards reported.