Digestion of phage λimm434 DNA with restriction endonuclease EcoRI yields 7 fragments. The shortest among them (1287 bp) contains the right part of the phage 434 immunity region and the phage DNA portion proximal to it. The complete primary structure of this fragment has been determined using the chemical method of DNA sequencing. Hypothetical amino-acid sequences of proteins coded by the cro gene of phage 434 and the cII gene of phage λ, as well as NH2-terminal amino-acid sequences of the cI protein of phage 434 and the O protein of phage λ, have been deduced solely on the basis of the DNA sequence. The fragment studied contains also the pRand probably Prmpromoter and the oRoperator of phage 434. The sequence coding for them differs from the respective DNA sequence of phage λ. © 1979.