Poly(L-lactic acids) (PLLAs) were grafted to both ends of poly(ethylene oxide) (PEO) to produce bio-compatible amphiphilic PLLA-PEO-PLLA triblock copolymers. The self-assembling behaviors of two PLLA-PEO-PLLA copolymers in aqueous solutions were examined by Dynamic Light Scattering and Transmission Electron Microscopic techniques. PLLA-PEO-PLLA formed spherical micelles, whereas PLLA-PEO-PPO-PEO-PLLA pentablock copolymers were reported to produce vesicles. It is believed that the PPO segment within the PLLA-PEO-PPO-PEO-PLLA pentablock copolymers has a dominant role in the formation of vesicles. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2445-2451, 2009