The high-pressure structural behavior of the fluoroperovskite KMg F3 is investigated by theory and experiment. Density functional calculations were performed within the local density approximation and the generalized gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. In situ high-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 40 GPa using synchrotron radiation. We find that the cubic Pm 3̄ m crystal symmetry persists throughout the pressure range studied. The calculated ground state properties-the equilibrium lattice constant, bulk modulus, and elastic constants-are in good agreement with experimental results. By analyzing the ratio between the bulk and shear moduli, we conclude that KMg F3 is brittle in nature. Under ambient conditions, KMg F3 is found to be an indirect gap insulator, with the gap increasing under pressure. © 2007 The American Physical Society.