The methanogenic strain MM isolated from an anaerobic microbial community degrading p-toluene sulfonate showed optimal values of temperature and pH for growth equal to 37 degrees C and 6.3-6.9, respectively. The doubling times of the isolate grown on methanol, acetate, and methylamines under the optimal conditions were 8.8, 19.1, and 10.3-28.1 h, respectively. The growth of strain MM was observed only when the cultivation medium contained casamino acids or p-toluene sulfonate. The G + C content of the DNA of the isolate was 40.3 mol%. This, together with DNA-DNA hybridization data, allowed the new isolate to be identified as a strain of the species Methanosarcina mazei. The new isolate differed from the known representatives of this species in that it was resistant to alkylbenzene sulfonates and able to demethylate p-toluene sulfonate when grown on acetate.