Critical steps implicated in the polymorphism of Wangiella dermatitidis were found to be sensitive to calcium ion availability. When grown in a defined, synthetic medium under various pH and temperature conditions, two thresholds of calcium ion concentrations were identified: a lower concentration favouring non-polarized growth leading to multicellular form development and a higher concentration promoting polarized growth characterized by yeast budding or pseudo/true hyphal growth. The phenotypic transition of yeasts to multicellular forms or to hyphae was induced at both 25 and 37 degrees C in the wild-type strain by the addition of calcium to the synthetic medium adjusted to pH 2.5, which was otherwise not conducive to the production of either growth form. However, the calcium additions did not allow maintenance of polarized growth of yeasts or hyphae in a temperature-sensitive, cell-division-cycle mutant (wdcdc2) derived from the same strain and grown at 37 degrees C in the same medium adjusted to either pH 2.5 or 6.5. Instead these conditions allowed only the nonpolarized, multicellular form development associated with this conditional mutant cultured in rich media at the 37 degree C restrictive temperature for yeast bud formation. Results from experiments using the calcium chelator EGTA added to the synthetic medium supported these conclusions at neutral pH with both the wild type and the wdcdc2 mutant cultured at 37 degrees C. The results suggested that during infection different concentrations of calcium may be encountered by W. dermatitidis in different tissues, which might directly regulate its growth and polymorphism and indirectly its virulence depending on host conditions.