A mutant of Escherichia coli strain C has been isolated, called gro109, that blocks bacteriophage P2 propagation by interfering with late gene expression. DNA replication proceeds normally in P2+-infected gro109 cells, but late phage proteins are not made. Early P2 mRNA is made in normal amounts, but very little late mRNA can be detected. P2 mutants (P2 ogr) able to overcome the gro109 block have been isolated in which synthesis of late P2 mRNA and phage proteins Is restored. The gro109 mutation is closely linked to the cluster of ribosomal genes at 64 min and is recessive to the wild-type (gro+) allele. A P2 ogr mutation has been mapped on the left arm of the P2 genome, between the right-most known late gene (D) and the phage attachment site. P2 ogr can complement P2+ in gro109 cells, indicating that ogr codes for a diffusible product.