The primary septum that forms in yeast cells at cytokinesis consists of the polysaccharide chitin. Three chitin synthetases (Chs1, Chs2 and Chs3) have been identified in Saccharomyces cerevisiae. Cloning and disruption of the respective genes showed that Chs3 is responsible for the formation of a chitin ring at the base of an emerging bud and of chitin dispersed in the cell wall, whereas Chs2 catalyzes the synthesis of a chitin disc that completes the primary septum. Chs1 acts as a repair enzyme, replenishing chitin lost through excessive action of a chitinase that facilitates cell separation by degrading part of the septum. The major structural polysaccharide of the yeast cell wall is beta(1->3)glucan. The glucan synthetase complex is bound to the plasma membrane. By differential extraction of the membranes with salt and detergents two solubilized fractions have been obtained which are required, in addition to GTP, for glucan synthesis. Further purification of one of these fractions led to results that indicate a role for other proteins in the modulation of GTP stimulation. A G-protein system appears to function in the regulation of beta(1->3) glucan and cell wall formation in vivo.