The infrared GaAs injection amplifiers described have gains as high as 2000 and give output powers of 150 milliwatts when operating at 77°K and driven by a single mode. Output efficiencies of 68 percent differentiial and 50 percent overall can be obtained if the amplifier is driven by light of spectral width equal to the bandpass of the amplifier (30 A). Regenerative effects are minimized by coating the diodes with three-fourth wavelength coatings of SiO. Thresholds at liquid nitrogen temperature were raised by a factor of ten on some diodes after the coatings were applied. Experimental data obtained on gain, efficiency, spectra, beamwidth, and noise power of several amplifiers are summarized and presented. Copyright © 1966 by The Institute of Electrical and Electronics Engineers, Inc.