The frequency mixing of infrared signals with visible laser light in nonlinear crystals (here referred to as up-conversion) has been suggested as a possible means of detecting infrared signals by use of sensitive and fast visible wavelength detectors. Preliminary experiments are reported of the up-conversion of the CW 10.6 μ CO2laser by phase-matched difference mixing in HgS with the CW 0.6328 μ, He-Ne laser to produce 0.6729 μ which is detected with an -20 photomultiplier. Reasonable agreement with theory is obtained. No additional sources of noise due to up-conversion were observed, although the actual performance was limited by pump fluctuations. No attempt was made to produce a practical device competitive with existing photoconductive detectors. In principle this is possible but would require various improvements, which are outlined. As a preliminary to the above experiment we report the measurement of the nonlinear coefficient ESU in HgS for second harmonic generation (SHG) of the 10.6 μ laser to produce 5.3 μ.