(S)-2-Methyl-l,5-bis(l,3-dimethyl-6-lumazinyl)-1,5-pentanedione (3), a new metabolite of the dimeric form of 1,3-dimethyl-6-acyllumazine derivatives, was isolated from the swimming polychaete, Odontosyllis undecimdonta. The empirical formula of the metabolite was established as C22H22N8O6 based on its high resolution mass spectrum. The ¹H nmr spectrum showed signals of two aromatic protons, four N-methyl protons, and four other coupled proton signals which indicated the presence of a disubstituted C4 chain, leading to the conclusion that the structure of this compound was the dimeric structure of 1,3-dimethyl-6-acyllumazines. The stereochemistry of the chiral center in 3, as determined by asymmetric synthesis starting from (+)-β-citronellene (4), indicated it to have the (S)-configuration. A synthetic product (3) thus obtained was identical with the natural product in all respects (uv, ir, ¹H nmr, ms, tlc, and specific rotation).