ABSTRACTPhotosynthetic pigments of the green flagellatePseudoscourfieldia marina(Throndsen) Manton (Micromonadophyceae) are similar to those of the coccoidPycnococcus provasoliiGuillard; prasinoxanthin is the predominant carotenoid. Other organisms that possess prasinoxanthin also possess additional pigments not found in eitherP. marinaorP. provasolii.Uriolide, a xanthophyll previously described from the coccoid done URI 266G, was also found inMantoniella squamata(Manton et Parke) Desikachary,Micromonas pusillaManton et Parke andMamiella gilva(Parks et Rayns) Moestrup, all flagellate members of the Mamiellales, and the coccoid clone IV E5G. Other unidentified carotenoids were also present inM. squamata, M. pusilla, andM. gilva.These results suggest thatP. marinaand the coccoid organisms URI 266G and IV E5G may be related to the Mamiellales, and thatP. provasoliimay be more closely related toP. marinathan toM. squamata, M. pusilla, andM. gilva.