Molecular diversity of rumen archaea was analyzed by PCR amplification and sequencing of two 16S rRNA clone libraries prepared from the bovine rumen fluid using two different archaea-specific primer sets. The first library of 19 clones which was generated with primers D30 and D33, produced essentially two groups of sequences, one affiliated with Methanomicrobium mobile (21% of clones) and the other -- with the uncultured archaeal sequences from anaerobic digester, which are distantly associated with Thermoplasma (79% of clones). The second library of 25 clones, which was generated with primers 0025e Forward and 1492 Reverse, produced a higher degree of diversity: in addition to the previous two groups, with the M. mobile- (56%) and Thermoplasma-associated sequences (20%), four clones (16%) were identified as Methanobrevibacter spp. The remaining two sequences were associated with unidentified archaeal sequences from the rumen and swine waste. Phylogenetic placement of eight almost complete 16S rRNA sequences revealed the existence of a novel cluster of the rumen Euryarchaeota, which is not affiliated with the known methanogenic archaea.