The present invention relates to the crystalline, polymorphic Form X of the calcimimetic compound {4-[(1R,3S)-3-((R)-1-Naphthalen-1-yl-ethylamino)-cyclopentyl]-phenoxy}-acetic acid, to methods of preparation thereof, to methods of characterization thereof by single crystal X-Ray crystallography (XRC), X-Ray Powder diffractometry, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Solid State NMR spectroscopy and Differential Scanning Calorimetry (DSC), and to its use. The invention also relates to the preparation of Form X by crystallization from a saturated solution of {4-[(1R,3S)-3-((R)-1-Naphthalen-1-yl-ethylamino)-cyclopentyl]-phenoxy}-acetic acid in a C1-C6 alkyl alcohol, or alternatively by precipitation from a neutralized saponification reaction mixture following the alkaline hydrolysis of a C1-C6 alkyl ester of {4-[(1R,3S)-3-((R)-1-Naphthalen-1-yl-ethylamino)-cyclopentyl]-phenoxy}-acetic acid.