Hybrids of phages λ and ∅80 were made by selecting for recombination between the genes for host range (h) and immunity (i). The parental origin, λ or ∅80, of markers in the h-i region of the hybrids was determined; and the following order of markers was deduced: h(att, int) exo (β, N). All the genetic elements involved in the specificity of the chromosomal site of attachment of λ and ∅80 appear to be located in the above region. The different types of hybrids obtained with respect to the genes for exonuclease and β protein provide genetic evidence that exonuclease and β protein are distinct polypeptides rather than polymeric forms of the same peptides. © 1969.