Five strains of methanogenic archaea isolated from two extreme habitats had been characterized for their physiological and phylogenetical properties. The strains SMA21 and SMA17 were isolated from the permafrost soil of the Lena river delta, Siberia and the strains MC15, MC20 and MC21 were isolated from the anaerobic sediment of the subsurface lake of Movile Cave, Romania, which contains a chemoautotophically based ecosystem with several endemic species.The strains SMA21 and SMA17 were irregular cocci whereas strain MC15 formed sarcinalike cocci in aggregates. The strains MC20 and MC21 were rodshaped. All strains grew within a temperature range from 0 up to 44°C, a pH value of 6.2 to 9.9 and salt concentrations from 0.02 to 0.6 M NaCl. Some were also able to grow at lower pH values down to pH 4.8 for SMA21, SMA17 and MC21 and even down to pH 4.1 for MC15. The most of the strains grew also at higher temperatures up to 54°C except MC20. The substrates used for methane formation were H2/CO2, methanol and acetate for the coccishaped and H2/CO2 and formiate for the rodshaped strains. MC20 could also use 2-propanol. Trypicase peptone and yeast extract were not needed for growth by any strain.The 16S-rRNA fragment sequences of the strains MC20 and MC21 could be allocated to the genus of Methanobacteria and the sequences of the strains MC15, SMA17 and SMA21 to Methanosarcina. The sequence of MC20 showed only 96.7% similarity to the sequence of the nearest cultivated relative Methanobacterium oryzae, which indicates for a discrete species. MC15 and MC21 showed a slightly higher similarity difference of 98.5% to Methanosarcina barkeri and 99.4% to Methanobacterium subterraneum, respectively. The strains SMA17 and SMA21 showed a high similarity to each other and to their nearest cultivated relative Methanosarcina mazei (99.8 and 99.9%), but with significant differences in their physiological properties.