AbstractThe content of total carotenoids and the ratio astaxanthin/idoxanthin ( = 3,3′‐dihydroxy‐β,β‐carotene‐4,4′‐dione/3,3′,4′‐trihydroxy‐β,β‐caroten‐4‐one) in varoius organs and tissues of one Atlantic salmon (Salmo salar, L.) reared indoors in a tank were analyzed after feeding ‘racemic’ ((3R,3′R)/(3R,3′S; meso)/(3S,3′S) 1:2:) astaxanthin (90 mg/kg feed) during one yera. Configurational analysis of astaxanthin was carried outviathe (−)‐dicamphanate derivative and that of idoxanthin after reaction with (+)‐(S)‐l‐(l‐naphthyl)ethyl isocyanate. Separation of all eight optical isomers of idoxanthin‐tricarbamate derivatives by HPLC is described. In salmon, enzymatic reduction of astaxanthin was found to be sterospecific leading to th (4′R)‐hydroxy group irrespective of the configuration at C(3′), thus resulting in four different stereoisomers of idoxanthin formed from (3R,3′