The E. coli rnc gene encodes the double-stranded, RNA-specific ribonuclease III (RNaseIII). A novel bacteriophage, gy1, was isolated, and its propagation in E. coli was shown to depend on the expression of RNaseIII in the cell. (a) gyl has a low efficiency of plating on rnc+ strains and a high efficiency of plating on a rnc-E. coli strain harboring the rnc 105 point mutation that renders its RNaseIII product inactive. (b) gy1 has a high efficiency of plating on rnc- strains in which the rnc gene is disrupted by a ΔTn10 insertion. (c) Plasmids harboring a rnc+ gene that were introduced into the rnc- strains described above reduced the efficiency of plating of gy1.