Total nucleotide sequence of plasmid pKYM which was isolated from Shigelia sonnei as a multicopy plasmid was determined by the Sanger's method. An open reading frame of 321 amino acid polypeptide (37 kilo dalton) was found in the sequence which corresponded to the rep gene found by the biological experiment. It was also determined that the minimal nucleotide sequence required to initiate replication was 133 bp present in the Hin dII-Hin dII fragment by constructing the plasmids with deletions around the replication origin.