Three-hundred-and-eighty-seven strains of Salmonella typhimurium obtained from successive generations of parent stock originating from three different rearing farms were characterized by phage typing and plasmid profiling. Seventy-six strains representing dominant types were selected for restriction enzyme analysis and colony hybridization. The main phage type on each of the three farms was 110. Plasmid profiling, however, allowed further subtyping. All but three isolates carried the serotype-specific virulence-associated plasmid. Restriction enzyme analysis showed variations in this plasmid as well as the presence of co-migrating plasmids of the same size. At each locality one or more clonal lines of S. typhimurium were reisolated from successive generations, indicating that the infections were persistent. Although house construction, sanitation and disinfection procedures, in addition to biosecurity in general, were improved significantly during the observation period, infection with S. typhimurium was not eliminated until eradication of the beetle Alphitobius diaperinus was complete.