We have cloned and sequenced four ribosomal protein genes from the extreme thermophilic archaeon Sulfolobus acidocaldarius P1. These genes code for proteins equivalent to L11, L1, L10 and L12 from Escherichia coli. The genes for the Sulfolobus L11, L1, L10 and L12 proteins are arranged in the same order as the equivalent genes in E. coli, i.e. L11-L1-L10-L12, and are transcribed as a single unit. Sequences resembling the consensus sequence for archaeal promoters have been detected upstream of the transcription initiation site. Transcription ends at several sites following a pyrimidine-rich region. The genes for proteins L11, L10 and L1 start with unusual initiation codons: GUG in the case of the L1 and L10 genes; and UUG in the case of L11. There are overlapping stop/start codons between the L11 and L1 genes, and between the L1 and L10, suggesting that the translation of the four genes might be coupled as in the bacteria.