Disclosed herein is a crystal of 1-ethyl-7-methyl-3-{ 4-[(3-methyl-3H-imidazo[4,5-b]pyridin-2-yl)oxy]phenyl} -1,3-dihydro-2H-imidazo[4,5-b]pyridin-2-one which is thermodynamically, chemically and physically stable. The crystal shows an X-ray powder diffraction pattern having characteristic peaks at interplaner spacings (d) of 13.59 ± 0.2, 6.76 ± 0.2, 9.22 ± 0.2, 7.88 ± 0.2, 6.21 ± 0.2, 6.13 ± 0.2, 5.73 ± 0.2, 4.64 ± 0.2, 3.79 ± 0.2 and 3.75 ± 0.2 Angstroms in powder X-ray diffraction. The crystal also shows an initial temperature of 222-224 °C of an endothermic behaviour caused by melting in DSC measurement (temperature increase rate 5 °C/min). The crystal is a phosphodiesterase 10A (PDE10A) inhibitor useful in the treatment of schizophrenia.
