Nu1 and A, the genes coding for bacteriophage lambda DNA terminase, rank among the most poorly translated genes expressed in E. coli. To understand the reason for this low level of translation the genes were cloned into plasmids and their expression measured. In addition, the wild type DNA sequences immediately preceding the genes were reduced and modified. It was found that the elements that control translation are contained in the 100 base pairs upstream from the initiation codon. Interchanging these upstream sequences with those of an efficiently translated gene dramatically increased the translation of terminase subunits. It seems unlikely that the rare codons present in the genes, and any feature of their mRNA secondary structure play a role in the control of their translation. The elimination of cos from plasmids containing Nu1 and A also resulted in an increase in terminase production. This result suggests a role for cos in the control of late gene expression. The terminase subunit overproducer strains are potentially very useful for the design of improved DNA packaging and cosmid mapping techniques.