Chemical analysis of the sponge Dysidea cf. arenaria from Irabu Island provided four new diterpenes 1-4. Their structures were elucidated by NMR and other spectroscopic analyses. All the metabolites retained the spongian skeleton and an isovalerate ester, but were different from those previously isolated from a specimen of Okinawa Island, implying geographic variation. The cytotoxicity of compounds 1-4 to NBT-T2 cells was evaluated and their IC50 values were 3.1, 1.9, 8.4, and 3.1 µM, respectively.