Research on the targeting of proteins to the cytoplasmic membrane of E. coli has mainly focused on the so-called 'general secretory pathway' (GSP) which involves the Sec-proteins. Recently, evidence has been obtained for an alternative targeting pathway in E. coli which involves the signal recognition particle (SRP). The constituents of this SRP pathway in E. coli are homologous to those of the well-characterized eukaryotic SRP pathway, which is the main targeting pathway for both proteins translocated across and inserted into the endoplasmic reticulum membrane. However, until recently, no clear function could be assigned to the SRP in E. coli. New studies point to an important role of the E. coli SRP in the assembly of inner membrane proteins.