In the course of screening for immunomodulators we have discovered a new cytochalasin produced in the fermentation broth of a Pestalotia sp. AB 1942R-114. The cytochalasin mycotoxins inhibit glucose transport and a wide variety of fundamental prokaryotic and eukaryotic cellular processes mediated by actin microfilaments.1) Compounds closely related to cytochalasin B bind to the glucose transporter and actin, while most other cytochalasins bind preferentially to actin sites.2) Cytochalasins that bind monomeric actin uncouple intrinsic actin ATPase activity from polymerization and inhibit both the rate of polymerization and the interaction of actin filaments in solution, resulting in inhibition of cellular motility and changes in morphology.1,3,4) In addition, cytochalasins interfere with immune function, with effects including inhibition of antigen-induced B-cell activation,5) inhibition of macrophage-lymphocyte interactions during antigen recognition,6) and suppression of the murine in vivo allogeneic antitumor response.7) We report the discovery, isolation, and structural identification of a new 10-phenyl[11] cytochalasin with in vitro immunosuppressant activity.