A total of 321 Salmonella typhimurium strains of bovine origin obtained in northern Japan during the period 1970–1979 were tested for drug resistance and detection of conjugative R plasmids. Three hundred and eighteen (99·1 %) of these strains were resistant to one or more drugs. The isolation frequency of multiply drug-resistant strains tended to increase year by year. Two hundred and thirty-seven (74·5%) of these resistant strains carried conjugative R plasmids. A total of 308 R plasmids including 174 (56·5 %) thermosensitive (ts) R plasmids were derived from the 237 drug-resistant strains, indicating that 71 (30·0%) strains have two different conjugative R plasmids in a single host cell. Of the 308 R plasmids examined for fertility inhibition (fi), 167 ts and 131 non-ts R plasmids were fi−. Of the 60 ts R plasmids examined for incompatibility, 50 were classified into H1 group and 10 into H2 group. Of the 52 non-ts R plasmids examined, 35 were classified into the Iα group and the remaining plasmids were untypable in our tests. Mercury resistance marker was found in about 20% of H1 R plasmids coding for multiresistance, and all of H2 R plasmids coded for resistance to tellurite. The clonal distribution of an S. typhimurium strain which carried an H1 R plasmid coding for resistance to six drugs and mercury was recognized in 1978 and 1979.