The elastic properties of a natural phlogopite have been studied by in-situ high-pressure X-ray powder diffraction experiments on the ID9 beamline at the ESRF, at room temperature. Several EoS models (Birch-Murnaghan, Vinet, Poirier-Tarantola) have been fitted to the experimental P-V data and the results obtained are presented and discussed. The third-order Birch-Murnaghan EoS, assuming V0 fixed at its experimental value, yields K0 = 49.7(±0.5) GPa, K’0 =8.59(±0.19), K’0 =-0.5953 GPa−1 (implied value). The axial compressibilities at room conditions, determined by the third-order Birch-Murnaghan EoS, result in βEoSa0=3.48(5), βEoSb0=3.2(1), βEoSc0=13.2(1) 10−3 GPa−1. Equilibrium thermodynamic calculations have been carried out to show how the new elastic parameters here reported affect the stability field of phlogopite.