Crude chitin synthetase (EC 2.4.1.16) from mixed membrane fractions of Blastomyces dermatitidis yeast and mycelial phases were examined. Enzyme from both phases exhibited sigmoidal kinetics indicating cooperative binding of substrate. Yeast and mycelial enzyme had essentially the same Km, pH optimum, temperature optimum, magnesium optimum and Hill coefficient. The major difference was that yeast enzyme was found in a mostly latent form which required proteolytic activation for maximal activity while mycelial enzyme was found in a fully active form. The data indicate that the difference in yeast and mycelial chitin synthetase from Blastomyces dermatitidis is one of state of activation rather than different enzymes.