Tryptophan pyrrolase was studied in partially purified extracts of Xanthomonas pruni. The dialyzed enzyme required both heme and ascorbate for maximal activity. Other reducing agents were able to substitute for ascorbate. Protoporphyrin competed with heme for the enzyme, suggesting that the native enzyme is a hemoprotein. The enzyme exhibited sigmoid saturation kinetics. Reduced nicotinamide adenine dinucleotide (NADH), reduced nicotinamide adenine dinucleotide phosphate (NADPH), nicotinic acid mononucleotide, and anthranilic acid enhanced the sigmoid kinetics and presumably bound to allosteric sites on the enzyme. The sigmoid kinetics were diminished in the presence of alpha-methyltryptophan. NAD, NADP, nicotinic acid, nicotinamide, nicotinamide mononucleotide, and several other related compounds were without effect on the activity of the enzyme. These data indicate that the activity of the enzyme is under feedback regulation by the ultimate end products of the pathway leading to NAD biosynthesis, as well as by certain intermediates of this pathway.