The properties of a photosensitizer for photodynamic therapy, mono-L-aspartyl chlorin e6 (NPe6), were investigated using phase-resolved fluorescence. NPe6 was analyzed in water solution at concentrations ranging from 3.13x10(-7) to 8.00x10(-5) M. The photophysical parameters of the lowest singlet excited state of NPe6 molecules were experimentally determined. It was confirmed that NPe6 molecules were in the isolated molecular state at concentrations below 1.00x10(-5) M. It was also confirmed that the fluorescence in this concentration range was ascribable to the electronic transition of isolated NPe6 molecules from the lowest singlet excited state to the ground state. At concentrations above 1.00x10(-5) M, some of the NPe6 molecules formed dimers in water solution, which caused a red shift of the fluorescence spectrum and an enhancement of fluorescence in the 700-750 nm wavelength region. Semiempirical molecular orbital calculation revealed that the sodium aspartate attached to the tetrapyrrole ring through the ethanoic group was remarkably bent with respect to the tetrapyrrole plane. This bending appeared to hinder the formation of NPe6 dimers at concentrations up to 1.00x10(-5) M.