The Corynebacterium glutamicum/Escherichia coli shuttle vector plasmid pZ1 was used to clone the S-(2-aminoethyl)-D,L-cysteine (AEC)-resistance gene from a lysine-excreting, AEC-resistant strain of C. glutamicum, the aspartokinase activity of which was released from feedback inhibition by mixtures of lysine and threonine or AEC and threonine respectively. A recombinant plasmid designated pCS2 carrying a 9.9-kb chromosomal insert that conferred AEC resistance and the ability to excrete lysine to its host was isolated. The aspartokinase activity of the pCS2-carrying strain was resistant towards inhibition by mixtures of lysine and threonine or AEC and threonine respectively. By deletion analysis the DNA region conferring AEC resistance to the host and feedback resistance to its aspartokinase activity could be confined to a 1.2-kb DNA fragment.