The carotenoids in two members of the Raphidophyceae (chloromonads) Gonyostomum semen and Vacuolaria virescens, and of two tentative members of this class Chattonella japonica and Fibrocapsa japonica were analyzed quantitatively and identified by methods including mass spectrometry and chemical derivatizations.Group I (G. semen and V. virescens) showed a similar carotenoid pattern, comprised of diadinoxanthin (54--60% of total carotenoid), dinoxanthin (8-17%), fl,//-carotene (7%) and heteroxanthin (7%), as well as neoxanthin (G. semen, 3%), an epoxidic monoacetate (G. semen, 12%), an epoxidic carotenol=9'-cis-diadinoxanthin? (V. virescens, 8%), an epoxidic diacetate (V. virescens, 2%) and vaucheriaxanthin 3,19-diacetate (V. virescens, 8%). Both chlorophyll c forms (c I and c2) were detected in V. virescens; ratio a/c~ + c 2, 20:1. This pigment composition resembles that of the Xanthophyceae.Group II (Chattonellajaponica and Fibrocapsajaponica) exhibited a different carotenoid pattern with fucoxanthin (40-68%), fucoxanthinol (3-42%), //,//-carotene (15-27%) and violaxanthin (1-2%) in common. C. japonica also contained zeaxanthin (1%). This carotenoid pattern resembles that of Chrysophyceae.Characteristic features Common to the carotenoids encountered are a high proportion of epoxidic carotenoids (78-86%), allenic carotenoids (24-82%), acetylated carotenols (18-81%) and acetylenic carotenoids (61-67%; Group I only).A scheme for the carotenoid biosynthesis that could lead to both observed patterns is proposed. The present analyses require a revision of phylogenetic schemes based on the photosynthetic pigments.