(32)P-labelled phosphoglucomutase was digested with trypsin after denaturation and two peptides were isolated that contained the bulk of the radioactivity bound to peptides. Both peptides appeared to derive from an identical section of the molecule. Peptic and subtilisin digests of the tryptic peptides were prepared. The resulting radioactive peptides were purified and their sequences studied. The presence of a single serine [(32)P]phosphate residue was clearly established. Difficulties in purification and low yields, especially of the tryptic peptide, prevented exhaustive sequence studies, but a tentative sequence is proposed as:Ala-Ile-Gly-Gly-Ile-Ile-Leu-Thr-Ala-SerP-His-Asx-Pro-Gly-Gly-Pro-(Asx(2),Gly)-Phe-Gly-Ile-Lys(where SerP represents serine phosphate and Asx represents aspartic acid or asparagine). The results do not support the presence of two serine phosphate residues in the denatured enzyme, but confirm previous results of a unique sequence around a single serine phosphate residue.