The x-ray spectrum of the Iowa State College synchrotron operating at 65 Mev. has been measured. The method consisted of the observation, in a magnetic cloud chamber, of the momentum of each member of electron pairs produced in the air filling of the chamber. Using the theoretical values of cross section for pair production and the results of recent experiments, the energy spectrum of the x-ray quanta was computed. Due to the pronounced forward directional characteristic of electron pairs produced at extreme relativistic energies, discrimination of one energy region with respect to another was avoided and no solid angle corrections were necessary . The results are in agreement with the energy spectrum predicted by the Bethe-Heitler theory.