We present a study of the nanoindentation behaviour of Zn1−xBexSe heteroepitaxial layers grown onto (001)-oriented GaP and GaAs substrates in the whole composition range 0≤x≤1. The alloy composition, applied peak load and orientation of the Berkovich indenter have been found to significantly affect the nanoindentation response of these films. In particular, the Young's modulus increases relatively steadily with beryllium composition (though discontinuities are observed at x0.2 and 0.7–0.9), whilst hardness reaches a maximum at x0.7–0.9. Hardness is anisotropic, as it depends upon the orientation of the side of the Berkovich pyramid relative to the [100] direction. The variation of hardness and Young's modulus are related to the percolative behaviour of Zn1−xBexSe. Our results show that the mechanical properties of ZnSe (in which the type of bonding is essentially ionic) are greatly improved by alloying with BeSe (in which the bonding is essentially covalent), though they remain well below those of usual III–V compounds such as GaAs or GaP.