For optically pumped FIR lasers the frequency of the pump beam is in close coincidence with an absorbing transition of the active medium. Saturable absorption results in an intensity‐dependent local refractive index, leading to self‐focusing or defocusing. The latter effect has been demonstrated experimentally in CH3F, in which a beam divergence of the order of several mrad was measured, in agreement with theoretical predictions. This phenomenon has important consequences in the design of high‐power FIR amplifiers.