Biological oxidation rates of CS2 with a mixed microbial culture obtained from a trickling filter were optimal with 3 mM CS2, pH 7, 30 degrees C and SO42- below 25 g l(-1). Degradation rates were 3.4 mg CS2/g(protein)min and 13.8 mg H2S/g(protein)min. The concentrations of intermediates (H2S, COS and S degrees) and the product (SO42-) of CS2 oxidation were measured. The biological oxidation was due principally to Gram negative bacteria.