Light induces beta-carotene synthesis in streptomycin-bleached Euglena gracilis Z. Light-adapted, chemostat-grown cells have up to 10-fold as much beta-carotene and 25% more protein than similarly grown, dark-adapted cells. Carotenogenesis does not occur under anaerobic conditions or in the presence of diphenylamine, cyanide, or cycloheximide. The blue portion of the spectrum (360-560 nm) is most active in initiating carotenogenesis. The level of cellular carotenoids is influenced by the type of carbon source and to some degree by pH. Phytofluence and zeta-carotene are present in dark-grown cells but not in cells grown aerobically in white light (360-1120 nm). These pigments, however, were present in cells grown in yellow or green light (above 486 nm) or in cells exposed to white light anaerobically. The carotenoids are localized in two types of structures at the light microscope level. A protoporphyrin was isolated from Euglena, and its role as a possible photoreceptor during carotenogenesis is suggested.