The relation between infrared absorption and mobility in gallium arsenide has been examined. The absorption theory of Haga and Kimura was used, as suggested by Vakulenko and Lisitsa, to calculate a theoretical drift mobility vs concentration curve for n-type gallium arsenide at 300°K. For concentrations above 1×1017 cm−3, this curve agrees well with that obtained from the Ehrenreich variational calculation. The Haga-Kimura theory was used also to predict that at temperatures between 77° and 300°K and concentrations above 1×1017 cm−3, the free-carrier absorption coefficient α in the far infrared (50–500 μm) takes the simple classical form α ∝Nλ2/μ, where N is the carrier concentration, λ the wavelength, and μ the drift mobility. This result suggests that far infrared absorption data may be used to directly calculate drift mobilities.