A previous study (Gustafson, G.L., and Thon, L. A. (1979) Biochem. Biophys. Res. Commun. 86, 667-673) demonstrated that Proteinase I from the cellular slime mold, Dictyostelium discoideum, was conjugated with phosphoryl moieties. This report describes a characterization of the covalent structure of these moieties. Essentially all of the phosphate associated with purified enzyme was released as a sugar-phosphate during mild alkaline hydrolysis. The sugar-phosphate was isolated from alkaline hydrolysates of Proteinase I by Sephadex G-25 chromatography and identified as the alpha-anomer of N-acetylglucosamine-1-phosphate. In separate experiments, involving acid hydrolysis of Proteinase I, it was shown that enzyme-phosphate could also be isolated as O-phosphorylserine. Based on the recovery of O-phosphorylserine from acid hydrolysates, it was concluded that the majority of the N-acetylglucosamine-1-phosphate residues in the proteinase were esterified to peptidyl serines through phosphoester bonds.