We have reported on a new type of thermotropic transition in synthetic diblock copolymer vesicles. Vesicles were self-assembled from a well-defined poly(trimethylene carbonate)-b-poly(L-glutamic acid) (PTMC22-b-PGA14, M(n) = 4.05*10(3) g mol(-1)) copolymer. PTMC(22) is a semi-crystalline hydrophobic block with a melting endotherm observed at 37 degrees C in the bulk copolymer and at 34-35 degrees C in the vesicular dispersions. This temperature transition, as observed by mDSC in the aqueous vesicular dispersions, arose from a variation of the PTMC chain packing inside the vesicular bilayer. A large range of vesicle sizes, from 100 nm to 5 mu m, was accessible by varying the experimental conditions to prepare the vesicles. Large vesicles were more sensitive to the melting transition due to a low membrane curvature and high hydrophobic block interactions. By many aspects, this transition presented strong analogies with the gel-to-liquid crystalline phase transition observed in liposomes. Vesicle budding and fission occurred when temperature was increased above the PTMC melting temperature and fusion events were observed when temperature was decreased. The fission process was certainly due to the creation of an excess area upon heating whereas fusion is due to membrane defects arising from variation in PTMC membrane packing. As a consequence, the size of the vesicles was found correlated to the crystalline state of the PTMC chains, its variation being fully reversible with the temperature.