A genotypically non-fimbriate (Fim−) strain of Salmonella typhimurium and a genotypically fimbriate (Fim+) strain derived from it by spontaneous mutation were compared for pathogenicity in mice. The two strains caused similar numbers of infections and deaths in groups of mice challenged by intraperitoneal inoculation, and nearly similar numbers in groups challenged by inoculation on to the conjunctiva, but the Fim+ strain caused many more infections (plus 26%) and deaths (plus 40%) than the Fim− strain when the inoculation was by mouth.