The phase diagram of boron nitride (BN) is similar to that of carbon, incorporating phases at high temperatures and pressures whose structures and physical properties resemble diamond. Cubic zinc-blende-structured BN is especially important because it is extremely hard-second only to diamond. Here we report the first measurement of the 300-K equation of state of this material to ultrahigh pressures (115 GPa), and obtain a zero-pressure bulk modulus of 369 ± 14 GPa. A theoretical equation of state derived from first-principles pseudopotential calculations yields a 300-K isotherm that agrees with our experimental results to better than 2.5% in volume and 2.0% in bulk modulus. The high-pressure Hugoniot (shock-wave equation of state) calculated from our equation of state for BN is in good agreement with existing shock-wave data. Our study illustrates the reliability of current experimental techniques (such as the ruby-fluorescence calibra-tion) and theoretical methods (pseudopotentials) for characterizing the behaviour of superhard, incompressible materials under high pressure.