In this study, we have demonstrated morphological and ultrastructural changes of growing hyphae as well as of constituting hyphal cells of A. fumigatus affected by a cell wall-active antifungal agent, micafungin, under varying treatment conditions. As in many other fungi, A. fumigatus contains 1,3-b-D-glucan in the cell wall as the major skeletal component [22,23]. Therefore, it is the rational speculation that inhibition by micafungin of fungal 1,3-b-D-glucan synthesis can induce abortive cell-wall formation in sus- ceptible fungi. Actually, the primary morphological changes produced by growth-inhibitory or -subinhibitory con- centrations of micafungin are those associated with the inhibition of normal cell wall formation. One of the most prominent changes was the development of numerous short branches on the lateral walls of hyphae. Although the exact reason remains to be answered, this abnormal hyphal growth might be a fungal response that compensates the inhibited apical growth of hyphae. Similar morphological changes were reported previously by Chiou et al.