Physical properties including the equation of state, elasticity, and shear strength of pyrite have been measured by a series of X-ray diffraction in diamond-anvil cells at pressures up to 50 GPa. A Birch-Murnaghan equation of state fit to the quasi hydrostatic pressure-volume data obtained from laboratory X-ray source/film techniques yields a quasihydrostatic bulk modulus K-0T = 133.5 (+/-5.2) GPa and bulk modulus first pressure derivative K-0T' = 5.73 (+/-0.58). The apparent equation of state is found to be strongly dependent on the stress conditions in the sample. The stress dependency of the high-pressure properties is examined with anisotropic elasticity theory from subsequent measurements of energy-dispersive radial diffraction experiments in the diamond-anvil cell. The calculated values of K-0T depend largely upon the angle between the diffracting plane normal and the maximum stress axis. The uniaxial stress component in the sample, t = sigma(3) - sigma(1), varies with pressure as t = -3.11 + 0.43P between 10 and 30 GPa. The pressure derivatives of the elastic moduli dC(11)/dP = 5.76 (+/-0.15), dC(12)/dP = 1.41 (+/-0.11) and dC(44)/dP = 1.92 (+/-0.06) are obtained from the diffraction data assuming previously reported zero-pressure ultrasonic data (C-11 = 382 GPa, C-12 = 31 GPa, and C-44 = 109 GPa).