Isothermal compressibility of hydrous wadsleyite has been measured for the first time by using in-situ X-ray diffraction in a diamond anvil apparatus at pressures up to 8.5 GPa of a sample which was synthesized at 15.5 GPa and 1300 ¿C under hydrous conditions. The hydrous wadsleyite sample included 2.5¿0.3 wt% of H2O as determined by secondary ion mass spectroscopy. The cell parameters of the present hydrous wadsleyite are fit to the orthorhombic symmetry within error. The linear compressibilities along a- and c-axes of the present hydrous wadsleyite are very similar to those of the anhydrous wadsleyite, but the liner compressibility along the b-axis of the present hydrous wadsleyite is ~15% larger than that of the anhydrous wadsleyite. Consequently, the calculated bulk modulus (K0) is 155¿2 GPa with the pressure derivative (K0') fixed to 4.3. This value is 5~11% smaller than that of anhydrous wadsleyite. On the basis of the above result, the seismic discontinuity around 400 km is discussed.¿ 1997 American Geophysical Union