SUMMARYA determination was made of the nucleotide sequence of the 7340-bp region of a ribosomal protein gene cluster of Halobacteriwn halobium, which is equivalent to the $10 operon of Escherichia coli. The sequence was analyzed with the codonpreference program deduced from the halobacterial codon usage table that showed a very high GC content of the third codon position. The sequence was comprised of a string of 13 tightly linked ORFs. Most of the ORFs were homologous with ribosomal protein genes (ORF1-ORF2-rp13-rp14-rp123--rp12-rps19-rp122-rps3-rp129-ORF11-rps17-rp114). The 13-gene string was preceded by three putative AT-rich promoter sequences. The order of the genes in H. halobium essentially agreed with that of the corresponding genes of E. coli (S10-operon), except for certain deletions or insertions of additional protein genes.