Caspofungin acetate (formerly known as MK-0991; trade name: Cancidas), a cyclic lipopeptide, is a new class of antifungal drug. It is a semi-synthetic analogue of pneumocandin B0 with improved water solubility, a significant limitation in the development of the echinocandin class as pharmaceuticals. Caspofungin acts by inhibiting the synthesis of β (1,3)-D-glucan, an essential component of the cell wall of susceptible fungi. Incubation ofA. fumigatuwith a single dose of caspofungin affected the same proportion of apical and subapical branching cells for up to 72 h.The cells at the active centers for new cell wall synthesis withinA. fumigateshyphae are killed when they are exposed to caspofungin.