Microbial biotransformation of coumarin (I) was undertaken using a battery of twenty five microorganisms. Among them, the fungus Cunninghamella elegans NRRL 1392 was the sole microbe that efficiently metabolized coumarin (I) into umbelliferone (II), 3, 4-dihydrocoumarin (V) and transcinnamic acid (VI). Umbelliferone was also biotransformed into 6, 7-dihydroxycoumarin (aesculetin, VII) and p-coumaric acid (VIII). On the other hand, warfarin (III) was biotransformed into 4`hydroxywarfarin (IX) and also dicoumarol (IV) was transformed into 4-hydroxycoumarin (X) by the same microorganism. The structures of the metabolites were established using only one property and spectroscopic techniques including melting points, 1 H NMR, 13 C NMR and mass spectroscopy. The cytotoxic effect of coumarin substrates and their metabolites was also investigated.