The Gram-negative bacterium Pseudomonas aeruginosa secretes many proteins into the extracellular medium. At least two distinct secretion pathways can be discerned. The majority of the exoproteins are secreted via a two-step mechanism. These proteins are first translocated across the inner membrane in a signal sequence-dependent fashion. The subsequent translocation across the outer membrane requires the products of at least 12 distinct xcp genes. The exact role of one of these proteins, the XcpA protein, has been resolved. It is a peptidase that is required for the processing of the precursors of four other Xcp proteins, thus allowing their assembly into the secretion apparatus. This peptidase is also required for the processing of the precursors of type IV pili subunits. Two other Xcp proteins, XcpR and XcpS, display extensive homology to proteins involved in pili biogenesis, which suggests that the assembly of the secretion apparatus and the biogenesis of type IV pili are related processes. The secretion of alkaline protease does not require the xcp gene products. This enzyme, which is encoded by the aprA gene, is not synthesized in a precursor form with an N-terminal signal sequence. Secretion across the two membranes probably takes place in one step at adhesion zones that may be constituted by three accessory proteins, designated AprD, AprE and AprF. The two secretion pathways found in P. aeruginosa appear to habe disseminate widely among Gram-negative bacteria.