A novel peptaibol, designated clonostachin, was isolated from cultures of Clonostachys sp. F5898 by HP-20 and silica gel column chromatographies and reverse-phase HPLC. The structure of clonostachin was determined by Edman and chemical degradations, positive ion FAB-MS, EI-MS, and NMR analyses. Clonostachin was a linear tetradecapeptide with an N-terminal acetyl group and a C-terminal sugar alcohol. Clonostachin inhibited ADP-induced aggregation of human platelets by 80% at 150μM. During the course of screening for blood coagulation inhibitors of microbial origin, we have found a novel peptaibol, clonostachin, as an active compound. In this paper, we describe the isolation, structure elucidation and biological activity of clonostachin.