The gene for the protein moiety of the light-driven chloride pump halorhodopsin (HR), hop gene, was sequenced and the primary structure of the protein derived thereof. The gene has a GC content of 67% and codes for 274 amino acids. A promoter structure, resembling that of the halobacterial 16S rRNA genes, is present and both a terminating stem and a loop sequence is found downstream of the TGA stop codon. A ribosomal binding site is located within the translated region. The HR protein moiety is processed at the amino terminus, as well as the carboxy terminus, yielding a dominant species of calculated Mr 26 961.