Four mutants of Chlorella vulgaris are described, which have been isolated after x-ray irradiation of the normal strain. They show blocks in the synthetic pathway of the carotenoids, along with a more or less completely blocked chlorophyll synthesis. The mixture of carotenoids formed by wild type chlorella either in light or in darkness contains mostly α- and /¿-carotene and xanthophylls (Fig. 1 a and b), whereas the mutants contain these pigments only in reduced amount or not at all. There appear instead several carotenoids of higher saturation, which are not found in wild type. Among them phytoene (Fig. 6), phytofluene (Fig. 7) and ζ-carotene (Fig. 8) were identified. Apparently they are piled up because they cannot be transformed into the ordinary end products of the synthetic chain. Mutant strain 5/871 synthesizes phytoene only. Neither phytofluene which is less saturated nor yellow pigments can be detected. The carotenoid mixture of strain 5/515 (Fig. 2) consists of phytoene, phytofluene and ζ- carotene. There is a complete block of xanthophyll synthesis. In strain 9 a xanthophylls are found (Fig. 5) but no α- or β-carotene or at least in minute amounts only. Instead the carotenoid mixture consists mostly of phytoene, phytofluene, ζ-carotene and one more carotenoid which was not yet identified. It has a maximum of absorption at 426 mμ. All mutants described so far have to be grown with glucose in darkness. They are killed by light. Another mutant strain (5/520) however was isolated which can be grown hetero- and autotrophically and shows striking differences in pigment formation depending on which condition was chosen (Fig. 3 a and 3 b). In the dark almost no chlorophyll and no xanthophylls, but phytoene, phytofluene, ζ-carotene and several unidentified carotenoids — among them possibly tetrahydrolycopene and lycopene — are produced. In the light strain 5/520 behaves like wild type with respect to chlorophyll and carotenoid synthesis. This may indicate that an enzymatically catalyzed step characteristic of wild type and missing in the mutant can be replaced here by the action of light.