A Methanosarcina species, designated strain ChGul, was isolated from goat feces; this is the first fully described pure culture of Methanosarcina obtained from feces. Antigenic fingerprinting suggests that isolate ChGul is a new immunotype. The mol% G + C content of DNA was 42.2%. Strain ChGul grew on methanol, methylamines, and acetate in a minimal salts medium. It grew on H-2-CO2 only after adaptation. Growth occurred as a milky-white suspension and contained cells mostly in doublets and quadruplets of irregular cocci; many cells contained phase bright spots typical of gas vacuoles. The isolate did not grow on formate, or CO2 plus isopropanol, ethanol, or acetone as substrates and did not produce methane from formate. The optimum growth temperature was 35-37-degrees-C, and optimum pH was 6.2-6.8. ChGul is unusually sensitive to sulfide and has low tolerance for NaCl. Optimal levels of total sulfide and NaCl for growth were 0.5 mM and 20-40 mM, respectively. Since ChGul requires adaptation for growth on H-2-CO2 and cannot use formate, it may be restricted to methylotropic or acetoclastic methanogenesis in the rumen, a function not observed in previously isolated rumen methanogens that use H-2-CO2 and formate. Our work suggests that improper NaCl and sulfide concentrations, and cell lysis, may have made isolation of rumen Methanosarcina difficult in the past. It also underscores the need to evaluate feed compositions and media components for most probable number studies, with respect to NaCl and sulfide levels, to understand the role of Methanosarcina in the rumen.