Two-hundred eighty-one strains of Pasteurella piscicida were collected from cultured yellowtail (Seriola quinqueradiata) in various districts of Japan between 1981 and 1983. Two-hundred sixty-two strains were resistant to combinations of chloramphenicol (CP), tetracycline (TC), ampicillin (APC), kanamycin (KM), nalidixic acid, furazolidone, and/or sulfamonomethoxine (SA). Transferable R plasmids were detected in 168 out of the 262 strains. The most common types of detected R plasmids were those encoded with resistance to CP, TC, and SA, and also those markers containing KM. Fortyone strains carrying APC-resistance had appeared first in 1982, and a transferable R plasmid was also detected for the first time.The R plasmids encoded with different resistance markers were constructed from common DNA sequences. Identical digestion patterns were observed in R plasmids detected from various areas. There was strong homology with these R-plasmid DNA. Consequently, the multiple drug-resistant strains of P. piscicida, carrying an R plasmid with the same DNA structure, were distributed in yellowtail culture farms in various areas.