The present communication deals with the genetic mapping of the temperate bacteriophage 59 immunity region. Morphological mutants of the c-type of the temperate bacteriophage 59 were isolated after UV-treatment of the phage. Complementation analysis of 36 c-mutants has shown that the bacteriophage 59 c-region consists of at least four genes responsible for the establishment of lysogenization. The two-factor crosses were used to determine the mutual positions (order and distances) of the c-mutations. The order of the c-genes is C1--C3--C2--C4. The three c-genes are localized in a short region of the phage 59 chromosome, but the fourth gene is located in the separate locus.