A measurement of the spontaneous fission neutron yield of natural uranium has been made by the activation of approximately 15001. of KMnO 4 solution in a sub-critical assembly containing over 5000 lb of natural uranium. By filtering a large volume of this solution, sufficient manganese activity is removed as MnO 2 by the Szilard-Chalmers effect to provide an accuracy of 1·5 per cent in the result. However, since the total efficiency of the MnO 2 extraction process was not known to this high degree of accuracy, comparison of the activation by Pu-Be sources of known strength superimposed over the uranium was made. This provided the spontaneous fission neutron yield of natural uranium in terms of the Pu-Be source calibration. A value of (1·532 ± 0·061) × 10 −2/sec per gramme was thus obtained.