Temperature-sensitive mutants of Alcaligenes faecalis that exhibited reduced growth at 42 C but grew well at 30 C were induced by N-methyl-N'-nitro-N-nitrosoguanidine. Four of the 5 mutants that were studied readily colonized the nasal mucosa of turkeys when given intranasally or in drinking water. Several mutants caused no signs of coryza or depression and produced minimal colonization of the trachea. Colonization of the nasal mucosa lasted 3 to 4 weeks and induced moderate levels of serum antibodies. When challenged with virulent isolates of A. faecalis 4 to 6 weeks after having been exposed to the mutants, unstressed turkeys were protected; turkeys subjected to water deprivation exhibited some colonization with the virulent strain and showed slight signs of coryza.