Spectra are presented of the responsivity and noise equivalent power (NEP) of liquid‐helium‐cooled InSb detectors as a function of magnetic field in the range 20–110 cm−1. The measurements are all made using a Fourier transform spectrometer with thermal sources. The results show a discernable peak in the detector response at the conduction electron cyclotron resonance (CCR) frequency for magnetic fields as low as 3 kG. The magnitude of responsivity at the resonance peaks is roughly constant with magnetic field and is comparable to the low‐frequency hot‐electron bolometer response. The NEP at the peaks is found to be comparable to the best long wavelength results previously reported. For example, we measure NEP=4.5×10−13W/Hz1/2at 4.2 K, 6 kG, and 40 cm−1. The InSb CCR will provide a much improved detector for laboratory spectroscopy, as compared with hot electron bolometers, in the 20–100 cm−1range.