Mono-(L)-aspartylchlorin-e6 (also known as Talaporfin, NPe6, MACE, and most recently LS-11) is a potent sensitizer for photodynamic therapy that is currently undergoing clinical trials. Using a combination of unambiguous partial synthesis from pheophytin-a and methyl pheophorbide-a, NMR spectroscopy, and single crystal X-ray diffraction, the structure of mono-(L)-aspartylchlorin-e6 is definitively shown to be the isomer in which the aspartyl residue is attached at the 15(2)-side chain position. This conclusion is contrary to earlier assumptions, but affirms the conclusions of a study based on NMR spectroscopy; a rationale for the unique formation of the 15(2)-aspartyl derivative is proposed.