In order to assess methanogen diversity in feces of pigs, archaeal 16S rRNA gene clone libraries were constructed from feces of the pig. After the amplification by PCR using primers Met86F and Met1340R, equal quantities of PCR products from each of the five pigs were mixed together and used to construct the library. Sequence analysis showed that the 74 clones were divided into ten phylotypes as defined by RFLP analysis. Phylogenetic analysis showed that three phylotypes were most closely affiliated with the genus Methanobrevibacter (46% of clones). The library comprised 55.4% unidentified euryarchaeal clones. Three phylotypes (LMG4, LMG6, LMG8) were not closely related to any known Euryarchaeota sequences. The phylogenetic analysis indicated that the archaea found in the libraries were all clustered into the Euryarchaeota. The data from the phylogenetic tree showed that those sequences belonged to three monophyletic groups. Phylotypes LGM2 and LGM7 grouped within the genus Methanobrevibacter. Phylotypes LGM4, LGM6, LGM8 and LGM9 grouped within the genus Methanosphaera. Other phylotypes grouped together, and formed a distantly related sister group to Aciduliprofundum boonei and species of the Thermoplasmatales including Thermoplasma volcanium and Thermoplasm acidophilum. Our results showed that methanogens belonging to the genus Methanobrevibacter were predominant in pig feces, and that many unique unknown archaea sequences were also found in the library. Nevertheless, whether these unique sequences represent new taxonomic groups and their role in the pig gut need further investigation.