The fission cross-section ratio 241pu:235U has been measured in the neutron energy range from 5 keV to 1.2 MeV with an accuracy between 3 and 4%. The neutron energy was determined by the time-of-flight method. Two identical gas scintillation chambers were used to detect the fission events. The present measurements were carried out at sufficiently fine energy intervals to show for the first time the detailed shape of the fission cross-section ratio. In contrast to the results of other authors, a distinct structure was found. A comparison with an evaluation by Davey of previous data shows a systematic difference in shape up to 10% between 30 and 400 keV. At higher energies the evaluation generally agrees with the new data except in the region between 900 and 1000 keV, where the values of this work indicate a significant dip in the cross-section ratio.