Two groups of haloalkaliphilic methanogenic archaea were dominating in enrichments from hypersaline soda lake sediments at pH 10. At moderate salt concentrations with formate or H₂ as electron donor, methanogens belonging to the genus Methanocalculus were enriched, while at high salt concentrations with methylated substrates, a group related to Methanosalsum zhilinae was dominating. For both groups, several pure cultures were obtained including the type strains AMF2^T for the Methanocalculus group and AME2^T for the Methanosalsum group. The Methanocalculus group is characterized by lithoheterotrophic growth with either formate (preferable substrate) or H₂ at moderate salinity up to 1.5–2 M total Na⁺ and obligate alkaliphilic growth with an optimum at pH 9.5. According to phylogenetic analysis, the group also includes closely related strains isolated previously from the low-salt alkaline Lonar Lake. The novel Methanosalsum group is characterized by high salt tolerance (up to 3.5 M total Na⁺) and obligate alkaliphilic growth with an optimum at pH 9.5. It has a typical methylotrophic substrate profile, utilizing methanol, methylamines and dimethyl sulfide (at low concentrations) as methanogenic substrates. On the basis of physiological and phylogenetic data, it is proposed that the two groups of soda lake methanogenic isolates are assigned into two novel species, Methanocalculus alkaliphilus sp. nov. (type strain AMF2^T = DSM 24457^T = UNIQEM U859^T) and Methanosalsum natronophilum sp. nov. (type strain AME2^T = DSM 24634^T = NBRC 110091^T).