Abstract The possibility of the production of fast β-rays by a charged thundercloud has been discussed by Professor C. T. R. Wilson, who has shown that such a cloud should spray upwards a stream of “runaway electrons” of energy as high as 5 × 109 electron-volts. The action of the earth’s magnetic field upon this stream should cause it to return to the earth at considerable distances from the cloud, supposing that the energy of the particles was not converted into γ-radiation by nuclear encounters on the way. A search for such particles in the region below a thundercloud has already been made by one of us, but without success. The fact that they are not produced in this region can be ascribed either to the field being too small and limited in extent or to the effect of nuclear stoppage. Although their presence would have strengthened the original suggestion, their absence does not invalidate Wilson’s view that upward-moving particles generated in the strong field within the cloud do exist. To test this suggestion directly, however, Is a more difficult problem, for the supposed electron spray as it returned to earth would be spread out over a wide region and its intensity at any particular point would not be large.