Calcineurin is a conserved Ca2+-calmodulin-activated, serine/threonine-specific protein phosphatase that regulates a variety of physiological processes, e.g., cell cycle progression, polarized growth, and adaptation to salt and alkaline pH stresses. In the pathogenic yeast Cryptococcus neoformans, calcineurin is also essential for growth at 37°C and virulence. To investigate whether calcineurin plays a role in the virulence of Candida albicans, the major fungal pathogen of humans, we constructed C. albicans mutants in which both alleles of the CMP1 gene, encoding the calcineurin catalytic subunit, were deleted. The C. albicans Δcmp1 mutants displayed hypersensitivity to elevated Na+, Li+, and Mn2+ concentrations and to alkaline pH, phenotypes that have been described after calcineurin inactivation in the related yeast Saccharomyces cerevisiae. Unlike S. cerevisiae calcineurin mutants, which exhibit reduced susceptibility to high Ca2+ concentrations, growth of C. albicans was inhibited in the presence of 300 mM CaCl2 after the deletion of CMP1, demonstrating that there are also differences in calcineurin-mediated cellular responses between these two yeast species. In contrast to C. neoformans, inactivation of calcineurin did not cause temperature sensitivity in C. albicans. In addition, hyphal growth, an important virulence attribute of C. albicans, was not impaired in the Δcmp1 mutants under a variety of inducing conditions. Nevertheless, the virulence of the mutants was strongly attenuated in a mouse model of systemic candidiasis, demonstrating that calcineurin signaling is essential for virulence in C. albicans.