Measurements of electron cyclotron resonance in n-type InSb at 4.2°K have been conducted using far-infrared laser emission lines at 337 and 311 μm from HCN and 119 μm from H2O. Well-resolved resonance lines were observed for each wavelength with the apparent effective mass dependent on the magnetic field strength. At 119 μm, a number of subsidiary maxima was observed on the low field side at ωc = ω(laser)/n where n = 1, 2, ..., 6, apparently due to intraband transitions to higher Landau levels. © 1969 The American Institute of Physics.