Here we present the in vitro selection of a novel ribozyme specific for Zn 2+- dependent catalysis on hydrolysis of a phosphorothiolate thiolester bond. The ribozyme, called the TW17 ribozyme, was evolved and selected from an artificial RNA pool covalently linked to a biotin-containing substrate through the phosphorothiolate thiolester bond. The secondary structure for the evolved ribozyme consisted of three major helices and three loops. Biochemical and chemical studies of ribozyme-catalyzed reaction products provided evidence that the ribozyme specifically catalyzes hydrolysis of the phosphorothiolate thiolester linkage. A successful ribozyme construct with active catalysis in trans further supported the determined ribozyme structure and indicated the potential of the ribozyme for multiple-substrate turnover. The ribozyme also requires Zn 2+ and Mg2+ for maximal catalysis. The TW17 ribozyme, in the presence of Zn 2+ and Mg2+, conferred a rate enhancement of at least 5 orders of magnitude when compared to the estimated rate of the uncatalyzed reaction. The ribozyme completely lost catalytic activity in the absence of Zn 2+, like Zn 2+-dependent protein hydrolases. The discovery and characterization of the TW17 ribozyme suggest additional roles for Zn 2+ in ribozyme catalysts.