Research for the "precursor" of chlorophyll in higher plants stimulated us to search for the "precursor" in Protista using similar techniques. Recently we have obtained an action spectrum for chlorophyll formation in Euglena (1). The action spectrum corresponds with the action spectrum for "greening" in etiolated Arena seedlings as determined by Frank (2) and with the absorption spectrum for protochlorophyll as given by Noack and Kiessling (3 a, b) and Koski and Smith (4). Studies by Smith (5) and Koski and Smith (4) indicate that protochlorophyll is the chlorophyll precursor as indicated in the action spectrum given by Frank (2).While the chlorophyll and carotenoid densities were being followed during this work, a decrease in carotenoid density was observed at high light intensities. Earlier, Strain (6) had suggested that the carotenoids are involved in chlorophyll formation based on the reports that the spectral bands which are most effective in chlorophyll formation are relatively ineffective in carotenoid formation (7) since a temporary depression of carotenoids but a steady increase of chlorophyll had been noted in the leaves of Phaseolus multiflorus on prolonged light exposure (7,8).Granick has reported a significant decrease in carotenoid density as Mg vinyl pheoporphyrin is transformed to chlorophyll in a mutant of Chlordla (9). A similar decrease in carotenoid density in oat seedlings has been observed at high light intensities and it has been suggested that the substance responsible for the carotenoid decrease is a prophyrin-type compound concerued with "esterifying" the porphyrin molecule and converting Mg vinyl pheoporphyrin to protochlorophyll, its phytol ester (10).In the present paper, an improved action spectrum is presented and, in addition, data on carotenoid formation and "destruction" are given. Also,