AbstractMaturation of bacterial viruses requires the formation of mature viral nucleic acid in the form found in progeny virus particles, the “packaging” of this viral nucleic acid within a capsule, and the assembly of “packaged” nucleic acid with the accessory structures necessary for infectivity. Recognition that the replicating viral nucleic acid is frequently in a form distinct from that found in mature particles has, in some instances, led to the postulation and formal description of presently unknown processes that must participate in the synthesis of the mature form. In at least some instances, packaging of the mature nucleic acid is clearly integrated with its synthesis.Examples involving the DNA bacteriophages T4, λ, and ϕ X174 and the RNA bacteriophages are presented to illustrate the