The morphology of a coccoid, methane producing bacterium growing on acetate was studied. The organism is capable of forming morphologically differentiated cells, nicrocysts, whose structure resembles that of bacterial surviving cells. The organism from peculiar macrocysts in the enrichment culture. In its other characteristics, the organism is similar to Methanococcus mazei. However, it clasibied as Methanosarcina, biotype 3, due to the characteristic formation of multicellular pseudococci.