From the marine sponge-derived fungus Stachylidium sp. six novel phthalide-related compounds, cyclomarinone (1), maristachones A-E (2-5), and marilactone (6), were isolated. The structure of compound 1 comprises a hydroxycyclopentenone ring instead of the furanone ring characteristic for phthalides and represents a new carbon arrangement within polyketides. In the epimeric compounds 5a and 5b the phthalide (=isobenzofuranone) nucleus is modified to an isobenzofuran ring with ketal and acetal functionalities. Biosynthetically the structural skeletons of cyclomarinone (1) and maristachones A (2), C (4), D (5a), and E (5b) are most unusual due to the presence of an additional carbon atom when compared to the basic polyketide skeleton. This special biosynthetic feature also holds true for the likewise isolated polyketide marilactone (6).