SUMMARY: High cell densities reached in cultures of Desulphovibrio desulphuricans (Hildenborough strain), grown in a medium of glucose, peptone, yeast extract and salts were due to the presence of a non-suIphate-reducing anaerobic contaminant whose presence was detectable only in special conditions. The relevance of this to previous studies on the Hildenborough drain is discussed and certain quantitative data are corrected. Studies on the nutrition of a strain of Desulphovibrio dmdphuricans (Postgate, 1951a) appeared to indicate that a medium of glucose, salts, peptone and yeast extract permitted growth to about 750pg. dry wt. cellslml. Peptone and yeast extract acted synergistically; peptone was the more important constituent since, with high concentrations of peptone, yeast extract could be omitted, but the reverse was not true. Various pure vitamins, amino-acids, etc., were tested for their ability to replace the complex nitrogenous supple- ments, and a mixture of cysteine, serine, ornithine and isoleucine had some growth-promoting effect. Grossman & Postgate (1958) showed that the cysteine in this mixture could be replaced by N%S, indicating that the cysteine functioned through its reducing properties and not as a nutrient. The present paper reports that the massive growth observed in the complex medium was partly due to the presence of an anaerobic contaminant in the stock strain used. The contaminant was only detectable in special conditions. METHODS