Experiments will be described which demonstrate that the product of purified “Qβ-replicase” is fully competent to serve both as a template and as a program for the synthesis of complete virus particles. The use of mutant RNA has permitted the demonstration that nucleic acid is the instructive agent in the replication process and hence satisfies the definition of a self-duplicating entity. Methods have been devised to examine the product both physically and biologically with a minimum of manipulation and with complete recovery of product and input template. A detailed analysis of every interval of synthesis has thus become possible. These technical advances have permitted us to demonstrate the existence of a latent period prior to the appearance of the first new complete infectious RNA molecules. Further, this latent period is accompanied by an eclipse of the input templates as infectious agents. The use of electrophoretic separation on acrylamide gels has yielded a detailed account of both templates and early product during the latent period, with the consequent identification of the intermediate stages. The resulting data and their implications for the mechanism of RNA replication will be discussed.