Host and viral deoxyribonucleic acid (DNA) metabolism in LPP-1-infected Plectonema boryanum was studied by equilibrium centrifugation in CsCl gradients. Approximately 50% of the host DNA is degraded to acid-soluble material between 3 and 7 hr after infection. Most of the acid-soluble product is reincorporated into viral DNA. Incorporation of exogenous 3 H-adenine into viral DNA can be detected very early after infection (within the first 2 hr), but the bulk of viral DNA synthesis occurs between 6 and 8 hr. Both the breakdown of host DNA and the synthesis of viral DNA require protein synthesis during the first few hours of infection.