The novel marine carotenoids, cucumariaxanthins A (1), B (2), and C (3), were isolated from the northern sea cucumber Cucumaria japonica. Their structures and absolute stereochemistries were determined to be (5S,6S,5'S,6'S)-(9Z,9'Z)-5,6,5',6'-tetrahydro-beta,beta- carotene-4,4'-dione for 1; (5S,6S,4'S,5'S,6'S)-(9Z,9'Z)-4'-hydroxy-5,6,5',6'-tetrahydro-beta, beta- caroten-4-one for 2; and (4S,5S,6S,4'S,5'S,6'S)-(9Z,9'Z)-5,6,5',6'-tetrahydro-beta,beta-car otene-4,4'- diol for 3 by extensive spectroscopic analysis and by the modified Mosher's method. Cucumariaxanthin C showed an inhibitory effect on Epstein-Barr virus activation in a short-term in vitro assay.