The variation of the lowest direct absorption edge (E 0) of ZnSe with hydrostatic pressure has been measured at room temperature with a diamond anvil cell for pressures up to the phase transition (13.5±0.2 GPa). The gap varies sublinearly with pressure. However, when the gap is plotted as a function of the relative change of the lattice constant (-Δa/a 0) it shows a slight supralinearly in contrast to most III–V tetrahedral semiconductors which retain a slight sublinearly even as a function of (-Δa/a 0). The experimental results are compared to theoretical calculations based (a) on the local density approximation combined with the self-consistent band calculations using the LMTO method (LD-LMTO) and (b) on a local empirical pseudopotential. Both calculations predict a sublinear dependence of E 0 as a function of (-Δa/a 0). The semiempirical Murnaghan's equation of state that was calculated with the LD-LMTO method, has also been tested by determining the lattice constant under pressure with an energy dispersive x-ray diffractometer.