Seven isoleucine, valine and leucine requiring mutants were isolated after treatment of Salmonella typhimurium LT-2 with nitrosoguanidine. Growth of six strains is stimulated by pantothenate. The mutant strains are classified into two groups. Mutants of group 1 have deficiency in activity of the α, β-dihydroxyacid dehydratase, and deficiency or alteration in the branched-chain-amino-acid aminotransferase. Group 1 is divided into polarity mutants, KA2, KA12 and KA13, and nonpolar mutants, KA1 and KA21, as judged by activities of the threonine dehydratase and the α, β-dihydroxyacid dehydratase. Activity of the reductoisomerase in the KA21 strain is also low. Mutants of group 2, KA6 and KA19, are deficient in activity of the α, β-dihydroxyacid dehydratase and show low valine permeability. The branched-chain-amino-acid aminoransferase of KA19 is also altered. The mutational site in each of the mutants is located in the ilv cluster. The evidence indicates that limited synthesis of α-ketoisovalerate is responsible for the multiple requirements of bacteria.