Near the right end of phage λ DNA, between gene Rz and the cos site, are 2050 bp of apparently non-coding DNA. We have cloned a λ DNA fragment containing this DNA into a plasmid and constructed a deletion, ω1, extending from a site within the Rz gene to a site about 560 bp from cos. This deletion could be recombined into viable λ phage at a frequency equal to that observed for the undeleted sequence. Recombinant phage λ carrying the ω1 deletion were demonstrated to have the same burst size and kinetics of phage production as undeleted A. The ω1 deletion can be used to extend the capacity of λ cloning vectors and to provide a region for the insertion of heterologous DNA which should exhibit controllable high level expression from the λ late promoter, p'_R.