Seventeen R factors originally found in community-isolated E. coli strains were transferred by in vitro conjugation experiments to three S. typhi strains and to J55, an E. coli J53 pro met derivative strain, in order to analyze R factor stability through 100 generations in liquid cultures at low bacterial density thus avoiding R factor retransfer. Four R factors were maintained in 100% of the S. typhi colonies tested after 100 generations. Four other R factors were completely or almost completely segregated, and nine R factors showed an intermediate segregation rate (average 50%). Resistance stability rate in J55 was high (above 90%) as was expected. Overall R factors in S. typhi tend to segregation and this may partially explain epidemiological data about low antibiotic-resistance incidence in S. typhi.