ABA-type triblock copolymers, poly{(γ-benzyl L-aspartate) (BLA)-block-ethylene oxide (EO)block-BLA}, with a controlled chain length were prepared. The copolymer film (ca. 0.01 mm thickness) was immersed into the water to create a thin, self-sustainable hydrogel membrane (0.01-0.02 mm thickness) that was largely deformable and tough in its equilibrated water-swollen state (water content: 58-83 wt %). Crossed-polarizing microscopy confirmed that the water-swollen membrane had a framework of giant crystalline domains 10 μ m-4 mm in size; as far as we know, crystalline domains large enough to observe by an optical microscope in the hydrogels have never been reported. During the water-swelling process, the triblock copolymers showed an α-helix-β-strand transformation of their PBLA blocks and long-range structural formation, which may have reinforced the hydrogel thin membrane, thus increasing its toughness.