Two genes, designated chsC and chsG were isolated from DNA libraries of the opportunistic fungal pathogen, Aspergillus fumigatus. The genes were characterized with respect to their nucleotide sequences and mutant phenotypes. The complete deduced amino acid sequences of chsC and chsG show that the products of both genes are Class III zymogen-type enzymes. A mutant strain constructed by disruption of chsC is phenotypically indistinguishable from the wild-type strain, but chsG− and chsC−chsG− strains have reduced colony radial growth rate and chitin synthase activity, conidiate poorly and produce highly branched hyphae. Despite these defects, the double-mutant strain retained the ability to cause pulmonary disease in neutropenic mice. However, in comparison to the wild-type strain, there was a decrease in mortality and delay in the onset of illness in mice inoculated with the double-mutant strain, which was associated with smaller and more highly branched fungal colonies in lung tissue.