This book is written about bioactive diterpenes from a soft coral Cespitularia sp. which was collected from the coast of Zamami Island, Okinawa, Japan. Four new alcyonolide congeners 1–4 were isolated from the first collection of the soft coral together with the known alcyonolide (5). In addition, methanol-free separation of the extract made from a second collection gave five new alcyonolide congeners 6–10 as well as alcyonolide (5). Alcyonolide was the major constituent of the ethyl acetate extract of both collections. These structures were determined by spectroscopic methods (1D, 2D NMR, IR and MS), chemical analyses and comparison with those of reported related compounds. All compounds are seco-type varieties of xenicin diterpenes and showed cytotoxicity (IC50 5.85–460 μM) against HCT116 celles.