Tetracyclines have been among the most widely used antibiotics worldwide. Plasmid-mediated tetracycline resistance among hospital strains of bacteria has continued to rise and of major concern has been the transfer of resistance to pathogenic organisms. Bacteraemia due to hospital acquired S. typhimurium has been a major cause of morbidity at Kenyatta National Hospital (KNH), hence the need to study drug susceptibility pattern of this organism. This study also characterized the tetracycline resistance genes using oligonucleotide probes. Ninety seven S. typhimurium strains isolated from patients at KNH were used. Agar dilution method was used to determine minimum inhibitory concentration (MIC). Plasmids were isolated from each strain and the different plasmid profiles were grouped by their molecular weights into 6 patterns. Out of 97, 87 (88%) strains were resistant. MIC ranged from 1 microgram/ml to 128 micrograms/ml. Genes encoding for tetracycline resistance were located on plasmids of molecular weights 65 MDa, 5.2 or both. Plasmid-encoded antimicrobial resistance is likely to spread to other pathogenic organisms, reduce our ability to treat the infection and increase the cost and duration of treatment.