Cyanuric acid hydrolase (AtzD) from Pseudomonas sp. strain ADP was purified to homogeneity. Of 22 cyclic amides and triazine compounds tested, only cyanuric acid and N-methylisocyanuric acid were substrates. Other cyclic amidases were found not to hydrolyze cyanuric acid. Ten bacteria that use cyanuric acid as a sole nitrogen source for growth were found to contain either atzD or trzD, but not both genes.