Bacteriophage T7 is unable to productively infect Escherichia coli strains carrying the sex factor F. T7 phage development, in terms of RNA and protein synthesis, was compared in T7-infected isogenic F- and F+ strains of E. coli. Slightly less T7 early mRNA and early protein were synthesized in F+ cells. In addition to the defect in T7 late protein production in F+ cells reported by others, significantly less T7 late mRNA was synthesized, about one-half of that produced in T7-infected F- cells. Moreover, host RNA synthesis was not completely inhibited. The protein-synthesizing ability of T7-infected F+ cells decayed much faster than that of F- cells both in vivo and in vitro. This faster decay appears to explain the failure of F+ cells to produce T7 late protein in vivo, even in the presence of a considerable amount of translatable T7 late mRNA. Therefore, it may not be necessary to postulate the involvement of specific translational discrimination against T7 late mRNA, although it appears that F-factor-mediated restriction of T7 involves changes in transcription as well as translation.