We characterized by antibiotic susceptibility, plasmid analysis, incompatibility grouping, and pulsed-field gel electrophoresis (PFGE) of XbaI- and SpeI-digested DNA 102 Salmonella enterica serovar Typhi (serovar Typhi) isolated from recent outbreaks of typhoid in three different parts of Kenya. Only 13.7% were fully susceptible, whereas another 82.4% were resistant to each of the five commonly available drugs: ampicillin, chloramphenicol, and tetracycline (MICs of >256 microg/ml); streptomycin (MIC, >1,024 microg/ml); and cotrimoxazole (MIC of >32 microg/ml). Resistance to these antibiotics was encoded on a 110-kb self-transferable plasmid of IncHI1 incompatibility group. The MICs of nalidixic acid (MIC, 8 to 16 micro g/ml) and ciprofloxacin (MIC of 0.25 to 0.38 micro g/ml) for 41.7% of the 102 serovar Typhi isolates were 5- and 10-fold higher, respectively, than for sensitive strains. Amplification by PCR and sequencing of the genes coding for gyrase (gyrA and gyrB) and topoisomerase IV (parE and parC) within the quinolone resistance-determining region revealed that the increase in the MICs of the quinolones had not resulted from any significant mutation. Analysis of genomic DNA from both antimicrobial agent-sensitive and multidrug-resistant serovar Typhi by PFGE identified two distinct subtypes that were in circulation in the three different parts of Kenya. As the prevalence of multidrug-resistant serovar Typhi increases, newer, more expensive, and less readily available antimicrobial agents will be required for the treatment of typhoid in Kenya.