Three novel compound polycitone A (1a) and polycitrins A and B (2 and 3) were isolated from the marine ascidian Polycitor sp. The structures of compounds 1a, 2, and 3 were established mainly on the basis of NMR spectroscopic data and, in the case of 1a, also by single-crystal X-ray diffraction analysis. The crystallographic analysis was performed on 1b, the penta-O-methyl derivative of 1a, both compound yielding poorly diffracting crystals of highly anisotropic shape. It required, in view of the dominating heavy-atom content of 1b, diffraction measurements with an intense rotating-anode X-ray source. Polycitone A and polycitrins A and B represent the first examples of two new classes of marine products which might biogenetically be close to the lamellarins. The penta-O-methyl derivatives 1b, was found to inhibit the growth of SV40 transformed fibroblast cells in a concentration of 10 μg/mL