(S)-6-(1-Hydroxypropyl)-3-methyllumazine (1) and (S)-6-(1-hydroxypropyl)-1,3-dimethyllumazine (3) were isolated from the marine polychaete Odontosyllis undecimdonta. In 1981, Cardellina and Meinwald isolated a 6-substituted lumazine derivative named leucettidine from the sponge Leucerra microraphis, proposing its structure as 6-(1-hydroxypropyl)-3-methyllumazine (1), but Pfleiderer revised it to 6-(1-hydroxypropyl)-1-methyllumazine (2) in 1984. We had previously reported isolating nine 6-substituted lumazine derivatives from O. undecimdonta, and in subsequent studies, two additional metabolites with the 1-hydroxypropyl side chain were obtained. The structure of one was identified as 1 (an isomer of leucettidine) and the other as 3 (its 1,3-dimethyl derivative). Their empirical formulas were established as C₁₀H₁₂N₄O₃ (1) and C₁₁H₁₄N₄O₃ (3) by high-resolution mass spectra, and structures were confirmed by comparison of UV and ¹H NMR data with racemic samples. The stereochemistry of both metabolites was determined to be the (S)-configuration by synthesizing optically active (1) and (3) from 4-O-benzyl-2,3-O-bis(methoxymethyl)-L-threose, followed by analysis of the (+)-MTPA esters of natural and synthetic products; their Rf values on TLC and ¹H NMR spectra were consistent, confirming the (S)-configuration of the chiral centers.