It was demonstrated that nitrate-reducing bacteria are able, under suitable condition, to synthesize nitrosamines. A nitrosation of aromatic secondary amines present in the medium resulted which was notable greater than that of aliphatic di-n-propyl-amine. In some cases more than half of the aromatic amine became nitrosated. This nitrosation occurred at such pH-values which do not permit a spontaneous reaction between secondary amines and alkali nitrites. The applicability of such a synthesis of nitrosamines by bacteria for human pathogenesis is discussed.