Plasmids are present in distinct copy numbers in exponentially growing cells. It means the frequency of replication initiation of plasmid DNA is defined in these cells. A large group of plasmids like pSC101 contains directly repeating, 18 to 21 base pairs sequences in the replication origin. The Rep protein encoded by the plasmid genome binds to the repeated sequences to function as an initiator of DNA replication. The protein has an additional function of a repressor to regulate its own synthesis by binding to the promotor region of its structure gene. Regulation of replication initiation by the Rep protein is discussed.