The virulent Yersinia enterocolitica strain 8081 killed BALB/c mice within 5 days of oral infection with a 50% lethal dose of log10 7.1, whereas an aroA mutant of 8081, YAM.1, and the plasmidless variant 8081c failed to kill mice. Unlike 8081, YAM.1 and 8081c did not persist or grow in the Peyer's patches, mesenteric lymph nodes, livers, or spleens of mice. Mice immunized orally with single doses of live YAM.1 were poorly protected against a lethal 8081 challenge, whereas mice immunized with three doses of YAM.1 were moderately well protected.