The energy spectrum of the beam of a four‐sector racetrack microtron has been measured here, using a high‐resolution double focusing beta‐ray spectrometer. For an energy gain per orbit of 750 keV, it was found that 50% of the integrated current in the eighth orbit (6 MeV) was contained in an energy interval of 34 keV and 90% of the beam current in a 73‐keV interval. This narrow energy spread (±0.3%) is of particular interest in the generation of submillimeter radiation and in the use of the microtron as an injector for high‐energy synchrotrons.