A new hexagonal cyclic en01 phosphate of 6-0 hydroxypropionyllumazine and its 3-methyl and 1,3 dimethyl derivatives were isolated from Odontosyllis undecimdonta. The marine swimming polychaete, Odontosyllis undecimdonta, was found to contain considerable amounts of 6-propionyllumazine derivatives. In efforts to isolate compounds related to lumazines from the same polychaete, a new type of cyclic phosphate of 6-0-hydroxypropionyllumazines was isolated, as discussed in the following.