Abstract It had long been supposed that new strains of viruses could be obtained only by the selection of spontaneously occurring variants. However a phenomenon discovered by Delbruck and Bailey (1) in 1946 furnished a powerful method of obtaining novel virus strains. When single bacterial cells are mixedly infected with two of the serologically related strains of phage, T2, T4 and T6, the virus progeny includes new hybrid types differing from both parental types. The new genotypes resulting from a recombination of parental characters persist through repeated subcultures. Genetic recombination in bacteriophages has been studied in mutants of T2 by Hershey and Rotman (2), in mutants of T1 by Hershey (3), and in hybrids of T2, T4 and T6 by Luria (4). What is undoubtedly the same phenomenon has been described by Burnet and Lind (5) with an animal virus.