Three novel compounds polycitone A (la) and polycitrins A and B (2 and 3) were isolated from the marine ascidian Polycitor sp. The structures of compounds la, 2, and 3 were established mainly on the basis of NMR spectroscopic data and, in the case of la, also by single-crystal X-ray diffraction analysis. The crystallographic analysis was performed on lb, the penta-0-methyl derivative of la, both compounds yielding poorly diffracting crystals of highly anisotropic shape. It required, in view of the dominating heavy-atom content of lb, diffraction measurements with an intense rotatinganode 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-0 methyl derivative lb, was found to inhibit the growth of SV40 transformed fibroblast cells in a concentration of 10 pg/mL.