1. Chorismate mutase, the first enzyme of the terminal biosynthetic pathway of phenylalanine and tyrosine has been purified 10 fold fromStreptomyces venezuelae. 2. The catalytic activity has a broad optimum between pH 6.6 and 7. The initial velocity data follow regular Michaelis-Menten kinetics with aKm of 3.8×10-4 M. The molecular weight of the enzyme is determined by gel filtration to be 55000. 3. l-Phenylalanine,l-tyrosine,l-tryptophan and the metabolites of the aromatic amino acid biosynthetic pathway shikimic acid, anthranilic acid, phenylpyruvic acid and p-hydroxy-phenylpyruvic acid are tested as potential modifiers of chorismate mutase activity. The activity of the enzyme is inhibited by none of these substances.