New L-threonine-producing strains of the genus Brevibacterium and Corynebacterium are obtained by separating a plasmid DNA from a microorganism of the genus Brevibacterium and Corynebacterium, inserting into the plasmid DNA a fragment of chromosomal DNA derived from a DNA-donor strain of the genus Brevibacterium or Corynebacterium resistant to a-amino-p-hydroxy valeric acid to obtain a recombinant plasmid DNA, incorporating the recombinant plasmid DNA into a recipient strain of the genus Brevibacterium or Corynebacterium which is sensitive to α-amino-β-hydroxy valeric acid and isolating a strain transformed to become resistant to α-amino-β-hydroxy-valeric acid.
By aerobically cultivating the new strains in an aqueous culture medium L-threonine may be obtained in high yields.