Activity of diaminopimelate decarboxylase in Escherichia coli strain W, growing in an aerated fermenter, was only slightly (14%) repressed by 2 mM-lysine when approximately equimolar diaminopimelate was present in the medium. Lysine alone caused 78% repression. Diaminopimelate did not interfere with uptake of lysine by growing organisms. Organisms grown in medium containing diaminopimelate, without lysine, had a decarboxylase activity 24% higher than organisms from minimal medium. The extent of repression by pyridoxine (56% when added to minimal medium) was decreased (to 31%) when diaminopimelate was also present in the medium. A diaminopimelate-requiring mutant, with limited ability to take up diaminopimelate, formed almost three times less diaminopimelate decarboxylase than did a diaminopimelate-requiring second-stage mutant that had an increased rate of transport of this amino acid. The internal concentration of diaminopimelate thus probably regulates the activity of the decarboxylase by induction. Lysine might not directly repress the enzyme but might give an apparent repression by restricting the biosynthesis of diaminopimelate. This restriction is probably not caused only by inhibition or repression of aspartokinase. Lysine and threonine together, though not singly, almost completely inhibited aspartokinase in vitro but caused less apparent repression of diaminopimelate decarboxylase than did lysine alone. Lysine plus diaminopimelate strongly repressed the lysine-sensitive aspartokinase (85%) without much affecting diaminopimelate decarboxylase formation, and pyridoxine repressed the decarboxylase without affecting aspartokinase.