The deletion plasmids, pRBH1 (1.5 MDa, kanamycin resistance, Kmr) and pUB110dB (1.5 MDa, Kmr), were obtained from pTB913 (2.9 MDa, Kmr, isolated from a thermophilic bacillus) and pUB110 (3.0 MDa, Kmr, from Staphylococcus aureus), respectively. All the nucleotide sequences of these deletion plasmids were determined. Replication origin regions of pRBH1 and pUB110dB contained, respectively, 63 base-pair inverted repeat and a large open reading frame, encoding RepB protein (235 amino acid residues). The nucleotide sequences were identical to each other except for one base in the center of the inverted repeat. Two copy number mutant plasmids, pRBHC3 and pRBHC7, were obtained from pRBH1. The mutation points were located at different positions in the RepB protein coding region (Gly to Asp for pRBHC3 and Gly to Glu for pRBHC7). RepB protein was shown to be involved in the copy number control of these plasmids.