A new methanogenic isolate termed JC3 is described. It is a Methanosarcina as demonstrated by morphologic (light and electron microscopy), physiologic, and DNA characteristics. It belongs to the species M. mazei according to DNA-base composition, DNA-DNA homology, and antibiotic sensitivity spectrum. JC3 can be considered a new M. mazei strain, since it differs from other well-characterized strains of this species in the following phenotypic features: conversion from packets to single cells, lamina formation, antigenic fingerprint of whole cells, and antigen pattern (immunoblot) of cell extracts.