High-pressure X-ray diffraction studies have been performed at room temperature on neptunium monoarsenide up to 53 GPa. This compound undergoes a phase transformation which starts around 25 GPa and is completed at 40 GPa. The high-pressure phase was indexed in the CsCl structure type. On pressure decrease, hysteresis to transform is observed down to 16 GPa. The bulk modulus B0 = 70 GPa and its pressure derivative B′ 0 = 6.2 have been calculated for the low-pressure phase by fitting the V( P) data to the Birch and Murnaghan equations.