We have found that the Aspergillus nidulans csmA gene encodes a novel protein which consists of an N-terminal myosin motor-like domain and a C-terminal chitin synthase domain (M. Fujiwara, H. Horiuchi, A. Ohta, and M. Takagi, Biochem. Biophys. Res. Commun. 236:75–78, 1997). To clarify the roles of csmA in fungal morphogenesis, we constructed csmA null mutants. The growth rate of the mutant colonies was almost the same as that of the wild-type strain, but hyphal growth was severely inhibited when a chitin-binding reagent, Calcofluor white or Congo red, was added to the medium. Moreover, morphological abnormalities in tip growth and septum formation were identified microscopically. Proliferation of intracellular new hyphae, called intrahyphal hyphae, which behaved as intrinsic hyphae, was the most striking phenotypic feature among them. These phenotypes were not suppressed when the only chitin synthase domain of csmA was expressed under the control of the alcA promoter, whereas they were suppressed when the intact form of csmA was expressed. Therefore, it was concluded that the product of csmA (CsmA) has important roles in polarized cell wall synthesis and maintenance of cell wall integrity and that the myosin motor-like domain is indispensable for these functions.