Methanogens are a group of anaerobic microorganisms of the domain Archaea that produce methane as an end product of metabolism. Most methanogens reduce carbon dioxide or ferment acetate to produce methane, but a small subset utilizes other substrates for methane production. Methanosarcina sp. strain MTP4, a methanogen originally isolated from a salt marsh, utilizes methanol, monomethylamine, dimethylamine, and trimethylamine as well as the volatile methylated sulfur compounds methanethiol (MT) and dimethylsulfide (DMS). With MT and DMS consumption, hydrogen sulfide, methane, and carbon dioxide are produced. In this study, closed system incubations of the methanogen Methanosarcina sp. strain MTP4 were conducted to quantify the stable sulfur (34S/32S) isotope fractionation during DMS consumption. Calculations show a sulfur isotope fractionation factor (epsilon) of about -8 per mil between the sulfur in DMS and that in the resulting hydrogen sulfide. The methanogens preferentially utilized DMS with the lighter sulfur isotope, thereby enriching unutilized DMS. These results provide further data that can be used to understand the biogeochemistry of sulfur and methane in anoxic environments.