Myelin basic protein of rabbit brain (Mr = 18,200) was initially freed of the bulk of the nonphosphorylated species (mainly component 1) by Cm-cellulose chromatography at high pH. The remainder of the protein was subjected to peptic digestion at pH 6.00, which resulted in specific, essentially complete cleavage at several bonds (Phe-44--Phe-45, Phe-87--Phe-88, Leu-109--Ser-110, and Leu-151--Phe-152) and partial cleavage at the Tyr-14--Leu-15 bond. Gel filtration of the digest through Sephadex G-25 (fine) yielded three fractions, the first containing primarily peptides 1-44 and 45-87, the second peptides 15-44, 88-109, and 110-151, and the third peptides 1-14 and 152-168. Each fraction was chromatographed on Cm-cellulose at pH 8.2, and the resulting subfractions and partially purified peptides were analyzed for phosphoserine and phosphothreonine. Materials containing significant amounts of the phosphoamino acids were subsequently chromatographed on Cm-cellulose at pH 4.65, and the analyses for phosphoserine and phosphothreonine were repeated. The resulting purified peptic phosphopeptides were identified by amino acid analysis and tryptic peptide mapping. Comparison of the maps with those of the unphosphorylated counterparts located the tryptic phosphopeptides. These were recovered and their identities were established by amino acid analysis. In those cases where the phosphopeptide contained 2 Ser residues, the position of the phosphoserine was established by aminopeptidase M digestion. Five phosphorylation sites were found: Ser-7, Ser-56, Thr-96, Ser-113, and Ser-163. Only a small fraction of these sites was phosphorylated in the total basic protein, with values ranging from about 2 (ser-113) to 6% (Thr-96). With the possible exception of Ser-56, these sites are not the ones that have been reported to be phosphorylated in vitro by cyclic AMP-dependent protein kinase.