A protein present on the cell surface of Bacillus licheniformis (Bl) NM105 was identified as an S-layer (OlpA in this paper), a protein present on many bacterial cell surfaces. Purification, SDS-PAGE and isoelectrofocusing showed one 94-kDa, slightly acidic (pI 6.5) protein band (defined as OlpA). The pure protein OlpA, has a tetragonal symmetry of its morphological subunits. Following Edman degradation, three 17-mer oligodeoxyribonucleotide (oligo) probes corresponding to the N-terminal sequence of Olpa were synthesized and used for gene cloning. The nucleotide (nt) sequence of the cloned gene (olpA) showed an ORF and encoded an 874 amino acid (aa) protein. In the promoter region of olpA, there appear to be -10 and -35 sigmaA-binding sites, as well as -10 and -35 regions specific for sigmaH. The existence of these two potential promoters suggests that OlpA would be produced during both the vegetative and sporulating stages of growth. The ribosome-binding site (RBS) sequence perfectly matched its consensus sequence, suggesting a high efficiency of translation of olpA. A typical 29-aa leader peptide, characteristic of secretory proteins in Bacilli, is present in the OlpA pre-protein sequence. In olpA, there are two stem-loop structures in tandem, downstream from the stop codon. These stem-loops are probably involved in prolonged olpA expression, by extending the half life of the mRNA.