Direct spectroscopic evidence has been obtained of microwave sidebands imposed on a monochromatic light beam by transmission through a single crystal of chromium tribromide at ferromagnetic resonance. Microwave frequencies above 13.5 Gc/sec may be used, and the most favorable optical wavelengths for this effect lie between 4750 and 5000 Å. This work has demonstrated modulation of a 4810‐Å beam by a 23.5‐Gc/sec microwave signal. This paper discusses the conditions under which the modulation may be observed, and how some of these may be optimized.