Phosphorylation of proteins catalysed by protein kinases is associated with central functions in growth and proliferation of the eukaryotic cell, and kinases are particularly important in the signal transduction pathways. Enterobacterial protein kinases are structurally and functionally different from eukaryotic protein kinases, and no prokaryotic kinase has so far been described implicating a direct role for this activity in virulence. Virulent Yersinia possess a common virulence plasmid that encodes a number of secreted proteins (Yops), of which YopH has protein-tyrosine phosphatase activity with a key function in the block of phagocytosis by the pathogen. Here we report that the virulence plasmid of Yersinia pseudotuberculosis encodes a secreted protein kinase (YpkA) with extensive homology to eukaryotic Ser/Thr protein kinases. Specific mutants of ypkA resulted in avirulent strains. Thus, YpkA is, to our knowledge, the first reported prokaryotic secreted protein kinase involved in pathogenicity, presumably by interfering with the signal transduction pathways of the target cell.