Inoculated orally, 16 Salmonella typhimurium strains belonging to 12 phage types varied greatly in their ability to kill 1-day-old chickens; variation was noted even between strains of the same phage type. Fourteen strains belonging to 11 food poisoning serotypes other than S. typhimurium were practically non-lethal when examined in this manner. All of them were lethal by the intramuscular route but some were more so than others. Two were more lethal by this route than one of the S. typhimurium strains that was highly lethal when given orally. With age, chickens rapidly became resistant to fatal infection with the food poisoning strains; given orally, a S. typhimurium strain killed 79% of 1-day-old chickens but only 3% of 2-day-old chickens. Of 2 specific poultry pathogenic strains, one, of S. gallinarum, was lethal by oral inoculation to chickens of all ages but the other, of S. pullorum, was only lethal to very young ones. Some salmonella strains, such as those of S. infantis and S. menston, were more efficient at infecting and colonizing the alimentary tract of chickens than were the more virulent S. typhimurium strains, the S. gallinarum and S. pullorum strains and a S. cholerae-suis strain.