An enzyme that is capable of removing the outer N-acetylglucosamine residues from phosphodiesters present on the high-mannose-type oligosaccharides of newly synthesized lysosomal enzymes has been described. This enzyme has been called an α-Nacetylglucosaminylphosphodiesterase, based upon its substrate specificity and on inhibitor studies. In this study it is demonstrated by the 18O enrichment method that the enzyme cleaves the CO bond rather than the OP bond, and therefore acts by a glycosidase type of mechanism. In addition, the enzyme has no significant activity toward α-N-acetylglucosamine 1-phosphate, and therefore requires an underlying phosphodiester for activity. In accordance with the IUB recommendations for enzyme nomenclature, it is therefore suggested that the enzyme be renamed α-N-acetyl-dglucosamine-1-phosphodiester N-acetylglucosaminidase (systematic name, 2-acetamido-2-deoxy-α-d-glucose 1-phosphodiester acetamidodeoxyglucohydrolase). For convenience, the trivial name phosphodiester glycosidase is proposed. Lysosomal α-Nacetylglucosaminidase also has a glycosidase type of mechanism but it is active toward α-N-acetylglucosamine 1-phosphate as well as phosphodiesters with outer N-acetylglucosamine residues.