The chemistry of the tapetum lucidum of the sea catfish (Arius felis) was investigated. The reflecting material consists of two components - one, a tapetal pigment consisting of oligomers of 5,6-dihydroxyindole-2-carboxylic acid, and another consisting of basic u.v. absorbing materials - which were studied. A main constituent of the latter was isolated by ion-exchange chromatography and precipitation as a flavinate. Chemical properties of this compound indicated its structure to be decarboxylated S-adenosylmethionine and this was confirmed by comparison with a synthetic sample. Thiamine was also identified as a minor constituent. A quantitative analysis of reflecting material purified by centrifugation showed that tapetal pigment, decarboxylated S-adenosylmethionine, and thiamine constituted 44, 17, and 2%, respectively, of the dry mass.