Recent studies have provided some insight to the overall characteristics of plasmid replication in bacteria. The ColE 1 and R6K plasmids replicate via a covalently-closed circular intermediate. Replication is initiated at a fixed origin and is unidirectional in the case of ColE 1 and bidirectional for R6K. In the case of the plasmid R6K. the bidirectional replication is asymmetric and sequential, proceeding from a fixed origin to a fixed terminus located approximately 20% of the R6K genome from the origin. RNA serves as a primer for plasmid DNA replication. The base composition and sequence of the 5'-terminus of the RNA segments in ColE 1 DNA have been determined. Finally, the properties of plasmid relaxation complexes and the possible role of these complexes in plasmid DNA replication are discussed.