Abstract: The consequences of two different mutations in the single essential actin structural gene of yeast (Saccharomyces cerevisiae) were studied. Both conditional-lethal actin mutants exhibit six phenotypes at the restrictive temperature: disruption of the asymmetric staining pattern of actin assembly; delocalized deposition of chitin on the cell surface; partial inhibition of secretion of the periplasmic protein, invertase; an intracellular accumulation of secretory vesicles; death of cells in the budded portion of the cell cycle upon prolonged incubation at the restrictive condition; and osmotic sensitivity. These results implicate actin in the organization and polarized growth of the yeast cell surface.