Penetration of wild-type T7 DNA into the host cell occurs in two steps. The phage particle ejects a few hundred base pairs of the left end of the genome into the host. Translocation of the remainder of the DNA is then coupled to transcription. In a normal infection, transcription-coupled translocation of wild-type T7 DNA is initiated at the major A1, A2, and A3 promoters for Escherichia coli RNA polymerase. At 37 degrees C, various deletion mutants lacking these three promoters grow at the same efficiency as wild-type T7 because the minor B promoter is efficiently transferred from the phage head into the cell. As the temperature of the phage infection decreases, the latent periods of (A1, A2, A3)- phages increase relative to that of wild-type T7; nevertheless, (A1, A2, A3)- phages have normal plating efficiencies at reduced temperatures. Lengthening of the latent period at low temperatures is due to a delay in transferring the complete (A1, A2, A3)- genome into the host cell. The (A1, A2, A3)- phages eject the leading end of their genome into the host, but at low temperature, insufficient DNA is transferred into the cell to allow RNA polymerase immediate access the B promoter. However, by an inefficient transcription-independent process, the B promoter eventually translocates into the cell. Mutant derivatives of (A1, A2, A3)- phages that have growth profiles at low temperatures similar to that of wild-type T7 have been isolated. The mutations allow both (A1, A2, A3)- and (A1, A2, A3)+ phages to translocate their entire genomes into the cell by a transcription-independent mechanism. The mutations are located in gene 16, a gene that encodes a component of the internal virion core. We postulate that gp16 is directly involved with the process of DNA translocation from the virion into the cell.