Visible light of moderate intensity inhibits growth, respiration, protein synthesis, and membrane transport in bakers' yeast and has a deleterious effect on membrane integrity. The results of this study indicate that these effects require the presence of cytochromes b and a/a3. The light sensitivities of growth rate and [14C]histidine uptake in wild-type rho+ Y185 and D225-5A strains of Saccharomyces cerevisiae were compared with those in a variety of mutants lacking cytochrome b or a/a3 or both; a close correlation was found between the presence of these respiratory pigments and photosensitivity. Thus, strain TL5-3C, a nuclear petite lacking cytochromes b, a, and a3, was resistant to light; strain GL5-6A, another nuclear petite having reduced amounts of cytochromes a and a3, was partially resistant; strains MB127-20C and MB1-6C, nuclear petites lacking only cytochrome b, were also only partially resistant to light; whereas mutants containing all three cytochromes but having their respiratory chain either nonfunctional (strain ZK3-6B) or uncoupled (strain 18-27/t12) were fully sensitive to light. Finally, an equal-energy, broad-band action spectrum for the light inhibition of growth and transport indicated that blue light (408 nm) was most effective; these wavelengths correspond to the Soret region of the cytochrome absorption spectrum. The results suggest, therefore, that the yeast cytochromes b, a, and a3 are the primary photoreceptors for the inhibitory effects of light and, perhaps, for other processes, such as the entrainment of biological rhythms in this species.