A new melanin biosynthesis inhibitor, melanoxazal, was isolated from the fermentation broth of Trichoderma sp. ATF-451 by successive purification procedures of carbon adsorption, ethyl acetate extraction and silica gel column chromatography. The inhibitor possesses a novel oxazole-containing structure with molecular formula, C8H9NO3. The structure was determined by means of NMR analyses to be (E)-4-(2'-formyl-3'-hydroxybuten-1'-yl) oxazole, which is related to melanoxadin. Melanoxazal inhibited melanin formation in the larval haemolymph of the silkworm, Bombyx mori; IC50 value = 30.1 micrograms/ml. Melanoxazal also showed a strong inhibitory activity against mushroom tyrosinase with IC50 value = 4.2 micrograms/ml.