The marine sponge Theonella mirabilis was reported to contain unusual depsipeptides, papuamides A-D, which showed HIV inhibitory and cytotoxic activities.2 During the collection cruise on R/V Toyoshio-maru of Hiroshima University to the Amami and Tokara Islands, we encountered a blue sponge closely related to T. mirabilis, 3 which exhibited a variety of bioactivities such as antifungal and protease inhibitory. We have already reported the isolation and structure determination of tokaramide A, a cathepsin B inhibitory linear peptide.3 Further examination of the extract afforded another cathepsin B inhibitor, miraziridine A (1).4 This work describes isolation and structure elucidation of miraziridine A (1).