The amidase from Rhodococcus rhodochrous J1, which hydrolyzes an amide to an acid and ammonium, was surprisingly found to catalyze the hydrolytic cleavage of the C-N triple bond in a nitrile to form an acid and ammonium stoichiometrically. The amidase exhibited a K m of 3.26 mM for benzonitrile in contrast to that of 0.15 mM for benzamide as the original substrate, but the V m a x for benzonitrile was about 1/6000 of that for benzamide. A mutant amidase containing alanine instead of Ser 1 9 5 , which is essential for amidase catalytic activity, showed no nitrilase activity, demonstrating that this residue plays a crucial role in the hydrolysis of nitriles as well as amides.