SUMMARY: H,S was evolved from soil treated with CCl, when moistened with sucrose and (NH,),SO, solution and incubated aerobically. H2S formation took place when the soil moisture was less than field capacity, and over a range of pH values from 5 to 8. The organism responsible was isolated and identified as Bacillus rnega- terium. Several strains of this organism reduced sulphate in well-aerated sterilized soil and liquid media but not in soil or liquid incubated anaerobically. The action of CCl, in fresh soil is to check or destroy certain fungi and bacteria which normally inhibit sulphate reduction by B. megaterium. Some of these organisms were isolated and shown to be sensitive to CCl, and to inhibit sulphate reduction by B. megaterium in sterilized soil. The isolates did not exhibit antibiotic action when grown in certain defined media. The reduction of sulphate in soil is known to be achieved by Desulphovibrio species, and although there have been reports of other micro-organisms able to reduce sulphate (Nadson, 1904; Nastukoff, 1895; Sawjalow, 1913; Shturm, 1948; Tanner, 1918), the widely held view is that only Desulphovibrio spp. have been shown with certainty to bring about this reduction (Bunker, 1936; Butlin, Adams & Thomas, 1949; Starkey, 1950; Starkey & Wight, 1945; Young, 1936). Desulphovibrio spp. are obligate anaerobes and utilize sulphate and other inorganic sulphur compounds as specific hydrogen acceptors during the oxidation of organic energy sources (Baars, 1930). The sulphur compounds are reduced to sulphides. Shturm (1950) claims to have obtained growth of Deswlphovibrio spp. under aerobic conditions in meat extract broth without added sulphates, but the reduction of sulphate under aerobic conditions has not been reported in the literature as far as is known to the author. The present author observed that soil after treatment with CCI, released H2S from sucrose + (NH,),SO, solution under what appeared to be well-aerated conditions. When this observation is considered in the light of existing literature the problem arises of how sulphate reduction can occur under such conditions. This paper describes experiments to discover whether anaerobic conditions were developing in the apparently well-aerated soil, the nature of the micro-organisms respon- sible for the sulphate reduction and the effect of CCl, on the process, METHODS The soils listed in Table 1 were crushed with pestle and mortar and passed through a nest of sieves of 3, 2, 1 and 0.5 mm. mesh. The particles remaining on the 3 mm. mesh were discarded and the particles on the 2, 1 and 0.5 mm. mesh collected and kept separate. The size of the particles of soil referred to