1,3-alpha-D-Glucan is an important component of the cell wall of filamentous fungi. We have identified a family of five 1,3-alpha-D-glucan synthase-encoding genes in Aspergillus niger. The agsA gene was sequenced and the predicted protein sequence indicated that the overall domain structure of 1,3-alpha-D-glucan synthases is conserved in fungi. Using RT-PCR and Northern blot analysis, we found that expression of the agsA gene and to a lesser extent also of agsE were induced in the presence of the cell wall stress-inducing compounds such as Calcofluor White (CFW), SDS, and caspofungin. Loss of agsA function did not result in an apparent phenotype under normal growth conditions but rendered the cells more sensitive to CFW. The induction of 1,3-alpha-D-glucan synthase-encoding genes in response to cell wall stress was not limited to A. niger, but was also observed in Penicillium chrysogenum. We propose that this response to cell wall stress commonly occurs in filamentous fungi.