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The phytopathogenic basidiomyceteUstilago maydisdisplays a dimorphic switch between budding growth of haploid cells and filamentous growth of the dikaryon. In a screen for mutants affected in morphogenesis and cytokinesis, we identified the serine/threonine protein kinase Cla4, a member of the family of p21-activated kinases (PAKs). Cells, in whichcla4has been deleted, are viable but they are unable to bud properly. Instead,cla4mutant cells grow as branched septate hyphae and divide by contraction and fission at septal cross walls. Delocalized deposition of chitinous cell wall material along the cell surface is observed incla4mutant cells. Deletion of the Cdc42/Rac1 interaction domain (CRIB) results in a constitutive active Cla4 kinase, whose expression is lethal for the cell.cla4mutant cells are unable to induce pathogenic development in plants and to display filamentous growth in a mating reaction, although they are still able to secrete pheromone and to undergo cell fusion with wild-type cells. We propose that Cla4 is involved in the regulation of cell polarity during budding and filamentation. [ABSTRACT FROM AUTHOR]