chsA and chsC are genes encoding class II and I chitin synthases of Aspergillus nidulans respectively. In a previous study, chsA chsC double mutants showed various growth defects, suggesting that their cell wall architecture was disorganized and their cell wall integrity diminished. Here, we constructed chsA chsC chsD triple mutants and chsA chsC csmA triple mutants to investigate the role of the class IV and class V chitin synthases, ChsD and CsmA respectively, in maintaining the cell wall structure of the chsA chsC double mutant. The former triple mutant grew a little slower than the chsA chsC double mutant, but the two showed similar phenotypes. In contrast, the latter triple mutant exhibited severe growth defects, particularly under low osmotic conditions. The levels of the csmA transcript of the wild-type strain and chsA or chsC single mutants were markedly elevated under low osmotic conditions, while that of the chsA chsC double mutants was high even under such conditions. These and other results suggest that the function of csmA is important for the maintenance of cell wall integrity and the polarized growth of the chsA chsC double mutant.