Isocoumarins have been isolated from a wide variety of microbial, plant and insect sources and have been shown to possess an impressive array of biological activities. Since the review by BARRY in 1963 (24), the number of known naturally occurring isocoumarins has increased dramatically. This increase is largely due to improvements in isolation procedures and structural analysis. Previous reviews have concentrated on fungal isocoumarins (293,294) and mycotoxic isocoumarins (301). This review lists over 160 naturally occurring isocoumarins. Leading references on isolation, structure elucidation, biosynthesis and synthesis are given in the accompanying Tables. The known natural isocoumarins are listed in the Tables according to the number and orientation of oxygen atoms on the benzenoid ring and by carbon substituents. For completeness, those isocoumarins bearing additional fused carbocyclic rings and those containing nitro gen substituents are included in separate Tables. It is hoped that by using these Tables in conjunction with the Formula Index, the Trivial Name Index and the Source Index the reader will be able to locate key references in the literature and gain an understanding of the fascin ating chemistry and action of naturally occurring isocoumarins.