Protein phosphorylation on tyrosine has been demonstrated to occur in a wide array of bacterial species and appears to be ubiquitous among prokaryotes. This covalent modification is catalyzed by autophosphorylating ATP-dependent protein-tyrosine kinases that exhibit structural and functional features similar, but not identical, to those of their eukaryotic counterparts. The reversibility of the reaction is effected by two main classes of protein-tyrosine phosphatases: one includes conventional eukaryotic-like phosphatases and dual-specific phosphatases, and the other comprises acidic phosphatases of low molecular weight. Less frequently, a third class concerns enzymes of the polymerase-histidinol phosphatase type. In terms of genomic organization, the genes encoding a protein-tyrosine phosphatase and a protein-tyrosine kinase in a bacterial species are most often located next to each other on the chromosome. In addition, these genes are generally part of large operons that direct the coordinate synthesis of proteins involved in the production or regulation of exopolysaccharides and capsular polysaccharides. Recent data provide evidence that there exists a direct relationship between the reversible phosphorylation of proteins on tyrosine and the production of these polysaccharidic polymers, which are also known to be important virulence factors. Therefore, a new concept has emerged suggesting the existence of a biological link between protein-tyrosine phosphorylation and bacterial pathogenicity.