The transcription mechanism during vegetative growth of coliphage Φ 80 was studied. The phage mRNA was assayed as the molecules hybridizable with “right-arm”, “left-arm” and “center” portions of the Φ 80 genome. The “left-arm” portion contains genes for head and tail formation, the “right-arm” portion contains genes for immunity, early functions and for lysozyme formation, and the “center” portion contains the “b2” region and some genes for integration. The molecules of mRNA corresponding to the “left-arm” were larger in size than those corresponding to the “right-arm” and “center” portions. In the late stages of phage development, the rate of synthesis of “left-arm”-mRNA was roughly six times higher than that of “right-arm”-mRNA. Chloramphenicol suppressed the synthesis of “left-arm”-and “center”-mRNA, but did not affect the synthesis of “right-arm”-mRNA. When defective lysogens carrying partially deleted Φ 80 prophages were induced by UV-irradiation, specific larger mRNA molecules, not found after induction of normal Φ 80, were synthesized. The synthesis of this mRNA could not be suppressed by chloramphenicol. This specific mRNA produced by deletion lysogens was shown to correspond to the “right-arm” and “center” portions of Φ 80 genome. It was also noted that the production of “left-arm”-mRNA was unexpectedly low, even if a considerable amount of the genetic material corresponding to head and tail genes was carried by the deletion lysogens. The synthesis of phage DNA was observed to be correlated with the length of genetic region preserved in the deleted prophage. The importance of a part of the phage DNA between end gene of tail formation and the “b2” region for the synthesis of “left-arm”-mRNA was suggested.