Osmotic shrinkage of giant egg-lecithin vesicles was observed by phase- contrast microscopy. The vesicles remained or became spherical when shrinking. Small and thick-walled vesicles formed visible fingers attached to the sphere. The water permeability of the single bilayer was found to be 41 micrometers/s. A variety of observations indicate that osmosis induces a parallel lipid flow between the monolayers of the bilayer, leading to a strong positive spontaneous curvature. They also suggest the formation of mostly submicroscopic daughter vesicles. The estimated coupling constant, 2 . 10(-6) mol/mol, is large enough to be biologically significant.