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Eudorina elegans was used to examine the response of an easily manipulated, multicellular organism to ultraviolet irradiation. The results indicate that E. elegans possesses an efficient photoreversal process. It is capable of complete reversal of ultraviolet induced damage sufficient to inactivate 99.99% of the colony-forming ability of the organism. Eudorina loses the ability to respond to visible light reversal of ultraviolet-induced damage exponentially with time. The half-life of this loss is about 10 h at 32° and about 20 h at 22°. Postultraviolet temperature of incubation influences the surviving fraction with fewer survivors at 22° than at 32°. The survival patterns of E. elegans suggest that a specific dark repair of ultraviolet-induced lesions may not occur, but that some repair processes take place during cellular deoxyribonucleic acid (DNA) synthesis.