Provided is a new crystal A of optically active 4-amino-3-(4-chlorophenyl)butanoic acid which is far better in stability, and a process for producing the crystal and a process for producing the crystal A comprising a step of heating the following crystal B in water having a pH of 3 to 9: crystal A: a crystal of optically active 4-amino-3-(4-chlorophenyl)butanoic acid which has diffraction peaks within the range of a diffraction angle 2θ of 8.7 to 9.4°, within that of 2θ of 12.2 to 12.8°, and within that of 2θ of 24.8 to 25.4° in a powder X-ray diffraction measurement of the crystal by use of the Cu—Kα wavelength; and crystal B: a crystal of optically active 4-amino-3-(4-chlorophenyl)butanoic acid which has diffraction peaks within the range of a diffraction angle 2θ of 20.8 to 21.4°, within that of 2θ of 26.7 to 27.3°, and within that of 2θ of 29.7 to 30.3° in a powder X-ray diffraction measurement of the crystal by use of the Cu—Kα wavelength.