1. The chloroplast of Spirogyra grows diffusively over its entire length even when irradiated only locally. Illumination of a disconnected chloroplast fragment also enhances the growth of other disconnected, non-illuminated fragments in the same cell. -2. When irradiated locally, the chloroplast becomes deformed to bring a greater part of it into the lighted area. Deformation caused by local illumination occurs only in the vicinity of the light-dark boundary. The chloroplast ribbon in this region shifts toward the lighted area not in parallel with the cell axis but obliquely to it. -3. Only light from the blue region induces the deformation. -4. The ability of the chloroplast to be centrifuged decreases in the illuminated region and increases in the shadowed region close to the light-dark boundary. -5. In a cell in which only the longitudinal half is illuminated, the chloroplast helix deforms to allow a greater part of the green ribbon to come into the illuminated half without changing its helical pitch.