The DNA fragment encoding the integrase and excisionase genes involved in site-specific recombination of staphylococcal bacteriophage phi 11 was cloned and sequenced. The int and xis genes and the recombination site, attP, were highly clustered in a 1.7-kilobase DNA fragment with the gene order attP-int-xis. The int and xis genes were transcribed divergently, with the int gene transcribed toward the attp site and the xis gene transcribed away from the attP site. The deduced Int is a basic protein of 348 residues with an estimated molecular weight of 41,357. In contrast, the deduced Xis is an acidic protein containing 66 amino acids with an estimated molecular weight of 7,621. The site-specific recombination system of phi 11 was compared with that of a closely related bacteriophage, L54a.