Irradiation of cells of Prototheca zopfii with blue light inhibited the respiratory capacity of the cells. The inhibition of respiration was correlated with a photodestruction of cytochrome c(551), cytochrome b(559), and cytochrome a3. Cytochrome c(549), cytochrome b(555), and cytochrome b(564) were unaffected by the irradiation treatment. The a-band of reduced cytochrome a was shifted from 599 to 603 nm by irradiation, an effect similar to that observed when methanol was added to nonirradiated cells. The presence of oxygen was required during irradiation for both photoinhibition of respiration and photodestruction of the cytochromes. Cytochrome a3 was protected against photodestruction by cyanide. Photodestruction of these same cytochromes also occurred when washed mitochondria of P. zopfii were irradiated.Blue and near-ultraviolet radiation are inhibitory or lethal to a wide variety of prokaryotic and eukaryotic microorganisms and to a number of tissues of higher plants and animals. Among the prokaryotes visible radiation and near-ultraviolet radiation have been reported to kill or inhibit the growth of the heterotropic bacteria Streptococcus salivarius (4), Escherichia coli (17,19,20) (8,31), the colorless algae Prototheca zopfii (14) and Astasia longa (8), and the yeast Saccharomyces cerevisiae (10,15,24,25,34), have also been reported to be inhibited by blue light. Various processes in cells of higher plants and animals have also been reported to be inhibited by blue or near-ultraviolet light, such as cell division (21, 42), respiration (35,33) and motility of sperm (41,28,29).In spite of such a large literature on the inhibitory effects of blue and near-ultraviolet radiations, there have been few definitive investigations focused on molecular mechanisms, with the