Five strains of methanogenic archaea (MT, MS, MM, MSP, ZB) were isolated from permanently and periodically cold terrestrial habitats. Physiological and morphological studies, as well as phylogenetic analyses of the new isolates were performed. Based on sequences of the 16S rRNA and methyl-coenzyme M reductase a-subunit (mcrA) genes all new isolates are closely related to known mesophilic and psychrotolerant methanogens. Both, phylogenetic analyses and phenotypic properties allow to classify strains MT, MS, and MM as members of the genus Methanosarcina. Strain MT is a new ecotype of Methanosarcina mazei, whereas strains MM and MS are very similar to each other and can be assigned to the recently described psychrotolerant species Methanosarcina lacustris. The hydrogenotrophic strain MSP is a new ecotype of the genus Methanocorpusculum. The obligately methylotrophic strain ZB is closely related to Methanomethylovorans hollandica and can be classified as new ecotype of this species. All new isolates, including the strains from permanently cold environments, are not true psychrophiles according to their growth temperature characteristics. In spite of the ability of all isolates to grow at temperatures as low as 1-5 degrees C, all of them have their growth optima in the range of moderate temperatures (25-35 degrees C). Thus, they can be regarded as psychrotolerant organisms. Psychrotolerant methanogens are thought to play an important role in methane production in both, habitats under seasonal temperature variations or from permanently cold areas.