The crystal structure of a novel esterase from Streptomyces scabies, a causal agent of the potato scab disease, was solved at 2.1 A resolution. The tertiary fold of the enzyme is substantially different from that of the alpha/beta hydrolase family and unique among all known hydrolases. The active site contains a dyad of Ser 14 and His 283, closely resembling two of the three components of typical Ser-His-Asp(Glu) triads from other serine hydrolases. Proper orientation of the active site imidazol is maintained by a hydrogen bond between the N delta-H group and a main chain oxygen. Thus, the enzyme constitutes the first known natural variation of the chymotrypsin-like triad in which a carboxylic acid is replaced by a neutral hydrogen-bond acceptor.