A new methanogen, designated as strain P2F9701a (= OCM 745), was isolated from a water sample of estuarine environment in Elrin Shi, Taiwan. Cells of strain P2F9701a were motile coccus (0.7 similar to1.1 mum) with tufts of flagella. Gas vacuoles were observed, and the protein cell wall was composed of S-layer protein subunit with M-r of 74,700. Cells catabolized formate and H-2+CO2 to produce methane, but not acetate, methanol, and trimethylamine. Strain P2F9701a grew in the range of 30-42 degreesC, with optimal growth temperature at 37 degreesC, but did not grow below 28 degreesC or above 42 degreesC. This estuarine isolate P2F9701a tolerated well the NaCl concentration between 0.02 and 1.03 M, and the optimal salt for growth was 0.17 M. Although phylogenetic analytic results indicated that P2F9701a belongs to the mesophilic, hydrogenotrophic marine methanogen of Methanococcus voltaei, the occurrence of gas vacuoles, tufts of flagella, eury-halotolerant and steno-thermotolerant characters of strain P2F9701a are different from mesophilic Methanococcus spp, that had been reported.