One of two 1.9kb HaeII segments of pSC101 is capable of autonomous replication. Replication of a plasmid containing the segment retains characteristics of the pSC101-type replication such as the dnaA-dependence, the copy number of the plasmid, the location of the origin, the direction of replication, and the incompatibility property. Analysis of deletion mutants revealed that a 1.65kb region within the HaeII segment contained functions sufficient for autonomous replication. A terminal 0.6kb segment on which the replication origin was mapped by an electron microscopic analysis of replicating molecules can replicate in the presence of a helper plasmid providing a traps-acting function and can express incompatibility toward pSC101.