Identification of a naturally occurring methyl-ester of phosphate, methyl-phosphorylcholine (methyl-2-(N,N,N trimethylamino) ethyl phosphate), in the eggs of the sea urchin S. purpuratus. A novel metabolite of choline, phosphorylcholine methyl ester, has been identified in the eggs of S. purpuratus wherein it is present at approximately 1 mM concentration. To the best of our knowledge, this is the first instance of a phosphoryl-methyl-ester to be observed in nature. The compound appears to be species specific, since it has not been observed in other species such as L. pictus and P. depressus. In S. purpuratus its distribution is confined to the ovary, eggs and embryos, and is absent from young animals following metamorphosis.