The P-type drug resistance factors RP4, RK2, R702, R68.45, and the N-type drug resistance factor R46 are transferred to Caulobacter crescentus at high frequencies. They are stably maintained and their antibiotic resistances are expressed. Experiments with RP4 have shown that intergeneric transfer of RP4 occurs with greater than 90% of the recipient cells in filter matings between Escherichia coli donors and C. crescentus recipients. Reciprocal matings with C. crescentus donors are less efficient, but still occur at a frequency of 10-1. C. crescentus strains maintain RP4 as a plasmid, are sensitive to RP4-specific phage, and segregate phage-resistant cells at a frequency of 10-4 to 10-5. The RP4 plasmid can be used in several ways: (1) the RP4 plasmid will promote chromosomal exchange between C. crescentus strains at frequencies ranging from 10-6 to 10-8; (2) RP4 will promote the transfer of nonconjugative colE1 plasmids from E. coli to C. crescentus; once transferred, the colE1 plasmid is stably maintained under nonselective conditions, can be transferred serially, and segregates independently from RP4; and (3) RP4 can be used to introduce transposons into the C. crescentus chromosome, providing the basis for additional genetic techniques.