The peptide Ac‐Ser‐Ala‐Gly‐Ala‐Gly‐Glu‐Gly‐Ala‐Gly‐Ala‐Gly‐Ser‐Gly‐OH was prepared with solid‐phase peptide chemistry. Before the removal of the peptide from the solid support, the alcohol side groups of the two serines were functionalized with an α‐bromo ester moiety to create a bifunctional initiator. This peptide‐based initiator was used in solution for the atom transfer radical polymerization of methyl methacrylate to yield a well‐defined ABA triblock copolymer, in which the poly(methyl methacrylate) end blocks had a number‐average molecular weight of 1.1 kg/mol (based on 1H NMR spectroscopy) and a polydispersity of 1.17. The aggregation behavior of this amphiphilic triblock copolymer was then investigated. Upon the suspension of the polymer in a mixture of tetrahydrofuran and water, followed by the removal of tetrahydrofuran, spherical aggregates were formed. By the application of different electron microscopy techniques, it was determined that these aggregates were polymersomes, presumably coexisting with large compound micelles. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6355–6366, 2005