The compressibility of wurtzitic boron nitride(wBN)taken in mixture with cubic BN has been measured at room temperature up to 66 GPa, using a diamond anvil cell and powder diffraction of synchrotron radiation. From the obtained pressure-volume relation forwBNthe isothermal bulk modulus ofB0=375±9 GPaand its first pressure derivative ofdB0/dp=4.9±0.7have been calculated indicating that this phase has nearly the same compressibility ascBN(B0=377±4 GPaanddB0/dp=4.1±0.2). Thermodynamic calculations using our findings onwBNequation of state have shown that wurtzitic boron nitride is metastable over the whole ranges of pressures and temperatures. ©1998 American Institute of Physics.