Chlamydomonas reinhardi is an excellent organism for research in photosynthesis for there are several mutant strains with impaired photosynthesis (21), as well as a number of mutant strains in which the pigments are visibly altered (25). In view of the recent investigations on the photosynthetic competence of these mutant strains (17, 19, 20) we felt it was desirable to obtain a complete description of their carotenoid pigments.In a preliminary study of the carotenoid pigments of C. reinhardi, Sager and Zalokar (26) separated 15 carotenoid fractions from their light-grown wild type strain. They were able to identify a-carotene, β-carotene and lutein. When grown in the dark, the wild type strain produced only 6 fractions, including the 3 identified in the light. In contrast, a pale green mutant strain (no. 95) which dies when grown either photosynthetically or heterotrophically in the light, produced only a-carotene and β-carotene in the dark.In the present study, 9 different carotenoid pigments could be identified in light-grown cultures of the wild type strain of C. reinhardi. Dark-grown wild type cultures contain 7 of these pigments, have less total pigment, and have a markedly decreased ratio of β-carotene/α-carotene. The latter 2 properties are also characteristic of the mutant strains described here.