The salt-tolerance gene HAL3 from Saccharomyces cerevisiae encodes a novel regulatory protein (Hal3p) which modulates the expression of the ENA1 sodium-extrusion ATPase (Ferrando et al., Mol. Cell. Biol. vol. 15, 1995, pp. 5470-5481). Hal3p contains an essential acidic domain rich in aspartates at its carboxyl terminus. We have isolated two cross-hybridizing genes from a genomic library of Candida tropicalis. One of the genes (CtHAL3) is a true homolog of HAL3 and it partially complements the salt sensitivity of a S. cerevisiae hal3 mutant. The activity of CtHAL3 was equivalent to that of an open reading frame (YKL088w) identified by genome sequencing of S. cerevisiae and with homology to HAL3. The other cross-hybridizing gene (CtCDC55) is a CDC55 homolog, encoding a protein with an internal acidic domain not present in the S. cerevisiae CDC55 product. Cdc55p is a regulatory subunit of protein phosphatase 2A and CtCDC55 complements the cold sensitivity of a S. cerevisiae cdc55 mutant. The presence of acidic domains in different putative regulatory proteins may suggest a role for this type of domain in molecular interactions.