Known as a prominent recombination path at high excitation densities, exciton-exciton annihilation (EEA) is evidenced in bulk hexagonal boron nitride by cathodoluminescence at low temperature. Thanks to a careful tuning of the exciton density by varying either the current or the focus of the incident electron beam, we could estimate an EEA rate of 2 × 10−6 cm3 s−1 at T = 10 K, the highest reported so far for a bulk semiconductor. Expected to be even stronger in nanotubes or atomic layers, EEA probably contributes to the luminescence quenching observed in low-dimensionality BN materials.