A non-selftransmissible multiple-copy plasmid, pEC3, was isolated from a phytopathogenic bacterium, Erwinia carotovora subsp. carotovora IFO3380. We localized the region required for replication in a 941-bp sequence by deletion analysis and subcloning of the region. The plasmid, which includes the minimal region required for replication and a kanamycin resistance-encoding gene, was compatible with that having ColE1 replication origin, but not with that containing the p15A replication origin. DNA sequence analysis of the 941-bp fragment indicated that the replication region of pEC3 is highly homologous to those of RNA (RNAI and RNAII) priming plasmids, such as ColE1 (pMB1), ColA, and p15A. The result suggests that the high-order structure of RNAI, rather than the homology of the primary sequence of the RNA, determines the compatibility.