The complete amino acid sequence of ribosomal protein H-S1 1 from the extremely halophilic archaebacterium Halobacterium marismortui is presented. This has been achieved by the sequence analysis of peptides derived by enzymatic digestions with trypsin, pepsin and Staphylococcus aureus protease, as well as by chemical cleavage with cyanogen bromide and o-iodosobenzoic acid. The protein consists of 155 amino acid residues and has a molecular mass of 17545 Da. Comparison of this sequence with other ribosomal proteins by the computer programmes RELATE and ALIGN showed that the C-terminal two-thirds of H-S 11 is homologous to the eubacterial ribosomal protein S 15 and that the N-terminal one-third of H-S 11 is possibly related to the N-terminal region of the eubacterial ribosomal protein S8. To explain this finding, possible genetic events during evolution, e.g. fusion or splitting of genes, are discussed. (Halobacterium marismortui)Halophilic ribosomal protein Evolution Amino acid sequence