The development cycle of the cyanophage AS-1 was studied in the host blue-green alga, Anacystis nidulans, under conditions that impair photosynthesis and under various light/dark regimes. Under standard conditions of incubation the 16-h development cycle consisted of a 5-h eclipse period and an 8-h latent period. Burst size was decreased by dark incubation to 2% of that observed in the light. An inhibitor of photosystem II, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), reduced the burst size to 27% of that of the uninhibited control, whereas cyanophage production was completely abolished by carbonyl-cyanide m-chlorophenyl hydrazone (CCCP), an inhibitor of photosynthetic electron transport. Dark incubation of infected cells decreased the latent period by 1-2 h and the eclipse period by 1 h, once the cultures were illuminated. This suggests that adsorption took place in the dark. Intracellular growth curves indicated that light is necessary for viral development. Infected cells must be illuminated at least 13 h to produce a complete burst at the same rate as the continuously illuminated control. Low light intensities retarded the development cycle, and at lowest light intensities no phage yield was obtained. AS-1 is highly dependent on host cell photophosphorylation for its development.