Plasmid profiles were investigated in 65 isolates of Salmonella typhimurium derived from animal outbreaks during the period of 1978 through 1983 in Japan. Incidence of plasmids, drug-resistance, and conjugative R plasmids were extraordinarily high in these isolates. This high incidence reflects the prophylactic and therapeutic use of antibiotics. Most isolates from diseased animals, cohabiting animals, and each farm showed the same or similar plasmid patterns. However, there was a difference in plasmid patterns within strains isolated from each of several animals. It may be that one or two plasmids were introduced or deleted in these strains, leading to the difference discerned in strains isolated from the same animal. It was also shown that during one epidemic, two strains of S. typhimurium were involved that could be distinguished by plasmid profile analysis. Our conclusion is that when S. typhimurium strains isolated from animals reared in limited areas exhibit identical or similar plasmid patterns, they are derived from the same source and that when strains isolated in a limited area exhibit quite a different plasmid pattern, these strains are derived from independent sources.