The role of photosynthesis in the infection of Nostoc muscorum by blue-green algal virus N-1 has been investigated. The adsorption rate constant (average value = 4.2 × 10-11 cm3/sec) is similar to that of bacteriophage. The one-step growth experiment revealed a latent period of 7 hr and a burst of 100-300 PFU/cell. No particular time in the viral development cycle seems sensitive to the withdrawal of light. Exclusion of light and 10-4 M CCCP, an inhibitor of electron transport, reduce the burst to 0-2% of controls. The burst decreases to 25% of controls when infected cells are treated with 10-6 M DCMU, an inhibitor of photosystem II.