Cdc24p is the guanine-nucleotide exchange factor for the Cdc42p GTPase, which controls cell polarity in Saccharomyces cerevisiae. To identify new genes that may affect cell polarity, we characterized six UV-induced csl (CDC24 synthetic-lethal) mutants that exhibited synthetic-lethality with cdc24-4ls at 23 degrees. Five mutants were not complemented by plasmid-borne CDC42, RSR1, BUD5, BEM1, BEM2, BEM3 or CLA4 genes, which are known to play a role in cell polarity. The csl3 mutant displayed phenotypes similar to those observed with calcium-sensitive, Pet- vna mutants defective in vacuole function. CSL5 was allelic to VMA5, the vacuolar H(+)-ATPase subunit C, and one third of csl5 cdc24-4ls cells were elongated or had misshapen buds. A cdc24-4ls delta vma5::LEU2 double mutant did not exhibit synthetic lethality, suggesting that the csl5/vma5 cdc24-4ls synthetic-lethality was not simply due to altered vacuole function. The cdc24-4ls mutant, like delta vma5::LEU2 and csl3 mutants, was sensitive to high levels of Ca2+ as well as Na+ in the growth media, which did not appear to be a result of a fragile cell wall because the phenotypes were not remedied by 1 M sorbitol. Our results indicated that Cdc24p was required in one V-ATPase mutant and another mutant affecting vacuole morphology, and also implicated Cdc24p in Na+ tolerance.