AMG-148, an oxathiolone-fused chalcone derivative, exhibited in vitro antifungal activity against several strains of human pathogenic yeast, with minimum inhibitory concentration values within the range of 1-16 μg ml(-1) and a fungicidal effect was observed at higher concentrations. Presence of major drug-effluxing membrane proteins Cdr1p, Cdr2p or Mdr1p, did not affect substantially the fungistatic activity of this compound against clinical Candida albicans strains. Studies on the mode of action revealed that AMG-148 inhibited chitin and β(1→3)glucan biosynthesis and was in vitro an inhibitor of β(1→3)glucan synthase. Inhibition of chitin biosynthesis was responsible for fungistatic activity, while the fungicidal effect was a consequence of disturbance of β(1→3)glucan synthase function. The chalcone derivative may be a useful lead compound for the development of novel antifungal agents.