We have studied P1 site-specific recombination by cloning a 6·5 × 10³ base EcoRI fragment (fragment 7) of P1 DNA into a λ vector and then asking whether that fragment can promote efficient recombination for λ markers that flank the fragment. Our results indicate that fragment 7 can reassort these markers very efficiently, and that this recombination can occur in the absence of the bacterial recA and recBC functions. The fragment 7 recombination system has been dissected by an analysis of deletion mutations into two components, a site (called loxP) that must be present in both partners in the recombination in order for recombination to occur, and a P1 gene (called cre), whose product is necessary for recombination. The location of the loxP site at the end of the P1 genetic map suggests that this site-specific recombination system is responsible for the lack of linkage between terminal P1 markers and therefore for the linearity of that map.