We have used a mating protocol to isolate recA-independent recombinants of pOX38, an F factor derivative, and the non-conjugative plasmid pMBO311. Plasmid pMB0311 is a derivative of pBR322 carrying a DNA insertion that contains IS121 and shows no extensive sequence homology to pOX38. Twenty-seven cointegrate molecules formed during independent mobilizations of pMB0311 by pOX38 were examined by restriction and Southern hybridization analysis. In general, there were two classes of recombinants. A minority class appears to have been mediated by IS121, resulting in the formation of cointegrate molecules containing IS121 at the junctions between the two plasmids. The majority class ( 23 27) apparently involved reciprocal recombination between sites on pOX38 and pMBO311. IS121 does not seem to be responsible for the formation of this type of cointegrate molecule, since similar structures were generated at approximately the same frequency during mobilization of control plasmids that do not contain IS121. We have localized the regions involved in this second class of recombination events and find that most ( 17 23) occur at or near oriV1, the primary replication initiation site of pOX38. Twelve of the cointegrate molecules showed identical restriction and Southern hybridization patterns demonstrating a preferred region on pMB0311 as well. This site was localized just distal to the tet genes, within a 640-base AvaI-PvuII segment in the pBR322 portion of the molecule.