Lambdoid phages are natural relatives of phage lambda. As a group, they are highly polymorphic in DNA sequence and biological specificity. Specificity differences have played a key role in identifying the specific sequences recognized by the N and Q antitermination proteins, the initiator O for DNA synthesis, the terminase system Nv1-A for cutting DNA during packaging, and the cI repressor protein. Variations that go beyond specificity differences are seen in packaging mechanism (headful in P22, specific cutting in lambdoid coliphages), in early control (terminator protein and phage-independent antitermination in HK022, phage-specific antitermination in lambda), in repression control (antirepressor operon in P22, absent in other lambdoid phages) and murein-degrading enzymes (transglycosylase in lambda, lysozyme in other lambdoid phages). Sequence comparisons indicate that recombination among lambdoid phages is frequent in nature.