We report on the synthesis of poly(L-lysine-b-caprolactone) block copolymers via an original and elegant approach whereby Z-L-lysine is first polymerized in the presence of allylamine and the resulting amino-terminated poly(Z-L-lysine) is then used in situ as macroinitiator for the polymerization of epsilon-caprolactone in the presence of tin(II) octanoate as catalyst. A range of poly(ester-b-peptide)s were obtained by varying the ratio of polypeptide to polycaprolactone units. After removal of the Z group of L-lysine units, the copolymers were soluble in water and their solution behavior was examined. They spontaneously form micelle-like objects in solution with sizes around 10 and 70 nm depending on the block lengths of the hydrophilic and hydrophobic sequences The smaller size particles would adopt a core-shell morphology while the larger particles would be more in line with a vesicle morphology.