We evaluated the potential for photodynamic therapy of a new photosensitizer, mono-L-aspartyl chlorin e6 (NPe6), whose tissue uptake and clearance are rapid. Firstly, NPe6 solution was exposed to a diode laser at 664 nm that fits the maximum absorption of NPe6. Immediately after laser exposure, the solution was subjected to electron spin resonance (ESR) to detect generation of activated oxygen species. With ESR, O2-, .OH, and 1O2 were detected in the NPe6 solution. O2- and .OH disappeared when superoxide dismutase was added to the solution. Next, the fundi of pigmented rabbits were irradiated with the diode laser after administration of NPe6. Histopathologically, there was necrosis of endothelial cells and vascular occlusion in the choroid, especially in the choriocapillaris. Retinal pigment epithelial cells were also injured. In contrast, the sensory retina was undamaged even after use of a large amount of dye. This new combination of NPe6 and diode laser at 664 nm enables us to occlude choroidal vessels efficiently by generating various activated oxygen species. Thus, photodynamic therapy with NPe6 is potentially useful in treating retinochoroidal involvement.