The extraordinary wave transmission through the afterglow of an rf‐excited, electrodeless, helium discharge has been investigated experimentally. The discharge approximates a low collision frequency plasma slab of relatively uniform electron density in a uniform magnetic field. Measurements were performed at 18.0 and 9.15 GHz. A transmission minimum occurs very close to the higher frequency cutoff and a large decrease in transmission occurs close to the lower frequency cutoff. The plasma cutoff positions are approximately independent of energy in the electron energy range from thermal energy to a few electron volts and of pressure in the pressure range from 0.08 to 2.5 Torr. The position of the transmission minimum coincides with the position of the emission peak previously observed.