The antimicrobial agent phomopsichalasin (1) was isolated from an endophytic Phomopsis sp. fermented on shredded wheat. The structure was determined by UV, IR, Nmr and mass Spectroscopy. Phomopsichalasin represents the first cytochalasin-type compound with a three ring system replacing the cytochalasin macrolide ring.A large number of cytochalasins have been isolated from a variety of filamentous fungi 1. The well-documented cell-level biological effects of the cytochalasins have been attributed largely to their ability to bind with actin filaments 1a. Two cytochalasins have also been found to act as HIV protease inhibitors 2. The cytochalasins are structurally comprised of an isoindolone unit fused to an 11-, 13- or 14- carbon macrocyclic ring1, 3. Cytochalasins are generally differentiated from one another according to whether they possess an isopropyl, an indolyl or a phenyl group attached at C-10 (Figure 1) 1-4. The novel compound phomopsichalasin (1) has been discovered which, while partially comprised of an isoindolone and a C-10 substituent, is strikingly different from the cytochalasins in having a 13-membered tricyclic system rather than a macrolide, a phenol substituent at C-10 rather than a phenyl group and an -OH moiety at C-3. It also represents a novel fused ring system. This paper describes the fermentation, isolation, structural elucidation and antibiotic activity of phomopsichalasin.