Mutant strains of microorganisms that require threonine and ones that require methionine have been reported in Neurospora (Horowitz, Bonner, Mitchell, Tatum, and Beadle, 1945), in Escherichia coli (Roepke, Libby, and Small, 1944), and in Bacillus subtilis (Burkholder and Giles, 1947). The finding that a monogenic Neurospora mutant that required both threonine and methionine could use the single amino acid homoserine (alpha-amino-gamma-hydroxybutyric acid) suggested that homoserine might be a precursor of threonine and methionine (Teas, Horowitz, and Fling, 1948). This evidence together with earlier-elucidated steps in Neurospora methionine synthesis (Horowitz, 1947) gives a general picture that can be verified and extended by the study of other threonine and threoninemethionine mutants. This paper reports experiments conducted on the nutrition and mutation of Bacillus subtilis mutants that require threonine or threonine plus methionine for growth. The biochemical implications with respect to threonine and methionine syntheses are similar to those from Neurospora. ; © 1950 American Society for Microbiology. Received for publication October 12, 1949. Oak Ridge National Laboratory operated by Carbide and Carbon Chemicals Corporation under Contract No. W-7405-eng-26 for the Atomic Energy Commission, Oak Ridge, Tennessee. ; Published - 93.full.pdf