Certain sponges contain microbial symbionts including bluegreen algae and bacteria and possess secondary metabolites, some of which may be of microbial origins. A Theonella sponge collected off Hachijo Island, from which we isolated bioactive cyclic peptides, also contained orange pigments possessing cytotoxic activity. Here we describe the isolation and structure elucidation of the pigments named aurantosides A and B, which are superficially reminiscent of the streptolydigins. The MeOH extract of the sponge (15 kg) was partitioned between water and ether, and the aqueous phase was extracted with n-BuOH. The n-BuOH phase was gel-filtered over Sephadex LH-20 with MeOH. The cytotoxic orange band was purified by reverse-phase chromatography to furnish aurantoside A (1, 1.3 × 10⁻⁴% yield based on the wet weight) and aurantoside B (2, 1.5 × 10⁻⁴% yield), both as orange amorphous powders. They are cytotoxic against P388 and L1210 leukemia cells (1, IC₅₀ 1.8 and 3.4 µg/mL, respectively; 2, IC₅₀ 3.2 and 3.3 µg/mL). A molecular formula of C₃₆H₄₆Cl₂N₂O₁₅ was established for aurantoside A by FAB mass and NMR spectral data, as well as combustion analysis. A conjugated hexaene (C8-C20) was inferred from COSY, HMQC, and HMBC spectra, with two chlorine atoms on C17 and C19 and all-trans double bond geometry. Three sugar units were identified: xylopyranose (sugar I), arabinopyranose (sugar II), and 5-deoxypentofuranose with a methoxy group on C2''' (sugar III). The structure also contains a tetramic acid moiety, and structure 1 was fully consistent with FAB-MS/MS data.