The sequence of 1360 base pairs of the P22 P_L operon was determined, linking a continuous sequence from P_L through abc2. P22 mutants bearing deletions in the sequenced region were constructed and tested for their phenotypes. Plasmids were constructed to express P_L operon genes singly and in combinations from P_lacUV5. Two previously known genes, 17 and c3, are located within this sequence. In addition, three new genes have been identified: ral, kil and arf. Genes ral and c3 are homologous, as well as functionally analogous, to λ ral and cIII, respectively. P22 kil, like λ kil, kills the host cell when it is expressed. The two kil genes, although analogous in cell killing and map location, have no apparent sequence homology. The functions of the P22 and λ kil genes are unknown; however, P22 kil is essential for lytic growth in the absence of abc. Gene arf (accessory recombination function) is located just upstream of erf; it is essential for P22 growth in the absence of kil or other genes upstream in P_L. The growth defect of P22 bearing a deletion that removes arf is complemented by expression of either arf or the λ red genes from plasmids. P22 sequences that include the stop codon for 17 potentially form a small stem-loop structure; these sequences are nearly identical to λ sequences that contain the stop codon for ssb. In λ this potential stem-loop structure occupies a map position near the terminator t_L2b. Plasmids that include the potential P22 structure negatively regulate kil gene expression in cis.