A fiber-optic Mach-Zehnder interferometer was used for x-ray detection. A single-mode, polarization-preserving silica fiber is exposed to a high-flux x-ray beam (x-ray tube for diagnostic radiology, 30 kV, 16.8-keV average energy, 17-40-mA anodic current), modulated at f = 9 Hz by a chopper with steel blades. The energy absorbed produces a modulated temperature rise that induces a phase shift in the propagation of a He-Ne laser beam with respect to the unirradiated arm of the interferometer. We measured the linearity of the temperature rise with the energy released to the fiber and the linearity of the peak amplitude at frequency f in the power spectrum of the interferometric signal with the anodic current of the x-ray tube. A possible application of this technique to synchrotron radiation monitoring is discussed.