Abstract A series of novel C 32 C 36 polymethylhentriacontenes has been identified in near-surface sediments of the Black Sea. Hydrogenation of these components indicated that they possess the 5,8,14,24,27-pentamethyl-, 8,14,24,27-tetramethyl-, 8,14,24-trimethyl-, 8,14-dimethyl-, and 14- and 15-methylhentriacontane C skeletons, which was confirmed by synthesis of an authentic standard of the C 34 member. The dominant C 36 member of this series contains eight double bonds. The 13 C content of these polymethylhentricontenes indicated that they are probably biosynthesized by photoautotrophs and that blooming or bicarbonate pumping affected the 13 C content of the fixed C. Their structures suggest that they were biosynthesized by methylation of an unsaturated n -C 31 precursor at specific positions in a well-defined sequence. This biosynthetic pathway represents an alternative for the biosynthesis of isoprenoid-like components.