Crude chitin synthase extracted from young (24 h) hyphae ofChoanephora cucurbitarumandPhascolomyces articulosus, susceptible and resistant hosts, respectively, to the mycoparasite,Piptocephalis virginiana, was identified and characterized by measuring the incorporation of the substrate [14C]UDP –N-acetylglucosamine into chitin. The enzyme activity was mainly associated with the mixed membrane fraction. Properties of the enzyme preparation such as activation with proteases,N-acetylglucosamine, magnesium, inhibition with polyoxin D,Vmax, apparentKmvalue for UDP –N-acetyl-D-glucosamine (UDP-GlcNAc), and response to pH were examined. Enzyme activity from both fungi displayed basically the same features as the corresponding enzymes reported from other mucoraceous fungi. However, the two enzyme preparations (fromP.articulosusandC.cucurbitarum) differed from each other in their response to various proteases and storage at 4 °C. Enzyme preparation fromP.articulosuswas activated by all proteases, whereas theC.cucurbitarumpreparation was activated by acid protease, slightly activated by trypsin over a narrow concentration range, and was inhibited by neutral protease. Enzyme preparation fromC.cucurbitarumshowed a rapid decrease in activity within the first 5 h of storage at 4 °C and also exhibited relatively higher activity of endogenous proteases than that fromP.articulosus.