Salmonella typhimurium isolated in Bombay from fecal samples of 145 patients suffering from gastroenteritis (group 1) and from the cerebrospinal fluid, feces, or blood of 42 patients with systemic salmonellosis (group 2) were examined for the antimicrobial resistance and incompatibility groups of their R plasmids. Multiple drug resistance was encountered in 88.9% of the isolates from group 1 and in all the isolates from group 2. The resistance found was mainly against ampicillin, chloramphenicol, kanamycin, streptomycin, sulfonamides, and tetracycline. In addition to these resistances, a number of isolates were also resistant to sulfamethoxazole-trimethoprim and gentamicin. The overall isolation frequency of strains resistant to these last drugs was significantly higher in group 2. The drug resistance in 95.3% of the isolates from group 1 and in all the isolates from group 2 was plasmid mediated. Incompatibility grouping of the R plasmids and phage typing of the isolates indicated that a clone of S. typhimurium with phage type pattern 66/122/untypable carrying Tra- IncF1me, Tra- Inc1, and Tra- Inc2 plasmids was most prevalent in Bombay from 1978 to 1980, and examples of this clone, especially those resistant to sulfamethoxazole-trimethoprim and gentamicin, were most often responsible for severe septicemic infection. A majority of the remaining S. typhimurium isolates were untypable and harbored plasmids of groups IncC, IncF1me, IncFII, IncH1, IncH2, IncI1 and IncI2; these isolates were rarely associated with systemic infection.