Two new diterpene derivatives of hypotaurocyamine, agelasidine B (2a) and agelasidine C (3a), have been isolated from the Okinawan sea sponge Agelas nakamurai Hoshino. The structures of agelasidine B and agelasidine C were elucidated by interpretation of spectral data and chemical degradation experiments. The agelasidines show inhibitory effects on growth of microorganisms, contractile responses of smooth muscle, and enzymic reactions of Na,K-ATPase.