SUMMARY. A series of experiments was undertaken to investigate the infection dynamics of various doses of S. typhimurium in day-old and 14-day-old broiler chickens kept in isolators. The infections were followed quantitatively in ceca and ileum by enumerating the colony forming units (cfu) of the challenge strain. It was found that the inoculation of 10⁷ cfu of S. typhimurium to day-old chickens established stable cecal infection in all the animals for 35 days. For 14-day-old chickens, stable and lasting infections were seen with inoculation of 10⁹ cfu. Lower doses yielded more variable results, and the bacteria were rapidly eliminated from most birds, especially in 14-day-old inoculated chickens. Salmonella was found in spleen and liver 2–3 days postinoculation. Salmonella was cleared from both organs or reduced to very low numbers within 3 weeks.