The chitin synthase gene WdCHS1 was isolated from a partial genomic DNA library of the pathogenic polymorphic fungus Wangiella dermatitidis. Sequencing showed that WdCHS1 encoded a class II chitin synthase composed of 988 amino acids. Disruption of WdCHS1 produced strains that were hyperpigmented in rich media, grew as yeast at wild-type rates at both 25 and 37°C and were as virulent as the wild type in a mouse model. However, detailed morphological and cytological studies of the wdchs1Δ mutants showed that yeast cells often failed to separate, tended to be enriched with chitin in septal regions and, sometimes, were enlarged with multiple nuclei, had broader mother cell-daughter bud regions and had other cell wall defects seen considerably less often than in the wild type or wdchs2Δ strains. Disruption of WdCHS1 and WdCHS2 in the same background revealed that WdChs1p had functions synergistic to those of WdChs2p, because mutants devoid of both isozymes produced growth that was very abnormal at 25°C and was not viable at 37°C unless osmotically stabilized. These results suggested that WdChs1p was more responsible than WdChs2p for normal yeast cell reproductive growth because strains with defects in the latter exhibited no morphological abnormalities, whereas those with defects in WdChs1p were frequently impaired in one or more yeast developmental processes.