Using data from a partial protein sequence analysis of ribosomal proteins derived from the archaebacteriumMethanococcus vannielii, oligonucleotide probes were synthesized. The probes enabled us to localize several ribosomal protein genes and to determine their nucleotide sequences. The amino acid sequences that were deduced from the genes correspond to proteins L12 and L10 from therifoperon, according to the genome organization inEscherichia coli, and to proteins L23 and L2, which have comparable locations, as in theEscherichia coliS10 operon. Various degrees of similarity were found when the four proteins were compared with the corresponding ribosomal proteins of prokaryotic or eukaryotic organisms. The highest sequence homology was found in counterparts from other archaebacteria, such asHalobacterium marismortui,Halobacterium halobium, orSulfolobus. In general, theM.vannieliiprotein sequences were more related to the eukaryotic kingdom than to the Gram-positive or Gram-negative eubacteria. On the other hand, the organization of the ribosomal protein genes clearly follows the operon structure of theEscherichia coligenome and is different from the monocistronic eukaryotic gene arrangements. The protein coding regions were not interrupted by introns. Furthermore, the Shine–Dalgarno type sequences of methanogenic bacteria are homologous with those of eubacteria, and also their terminator regions are similar.Key words: archaebacteria, ribosomal proteins, evolution, gene organization,Methanococcus vannielii.