The in silico analysis of the amino acid sequences deduced from the complete genome sequence of <i>Staphylococcus aureus</i> suggested the presence of two protein tyrosine kinase activities, each split into two distinct polypeptides, respectively Cap5A1/Cap5B1 and Cap5A2/Cap5B2, like in some other Gram-positive bacteria. To check this prediction, the corresponding genes were cloned and overexpressed, and the four corresponding proteins were purified by affinity chromatography and assayed for phosphorylating activity in vitro. Individually, none of them was found to autophosphorylate. However, when Cap5B2 was incubated in the presence of Cap5A2 or, with a larger efficiency, in the presence of Cap5A1, this protein exhibited intensive autokinase activity, occurring selectively at tyrosine residues. On the other hand, whatever the protein combination assayed, Cap5B1 did not present any phosphorylating activity. In search of a possible role for the phosphorylation reaction mediated by Cap5B2, an endogenous substrate of this kinase was characterized. This substrate, termed Cap5O, is the enzyme UDP-acetyl-mannosamine dehydrogenase involved in the cascade of reactions leading to the synthesis of the bacterial capsule. It represents the first endogenous substrate for a tyrosine kinase activity so far identified in <i>S. aureus</i>. The analysis of its dehydrogenase activity showed that it was positively controlled by its phosphorylation at tyrosine.