Hall mobilities of germanium single crystals have been measured at a frequency of 9000 Mc over the temperature range 30°–300°K. A rectangular sample occupied the central part of a wall of a rectangular cavity, which was doubly degenerate in the TE101mode and in the TE011mode at a single resonance microwave frequency. The external magnetic field and the microwave field associated with one of the two modes gave rise to the other mode of oscillation. The theoretical analysis by Liu, Nishina, and Good was verified by measurements on an n‐type sample having a room temperature resistivity of 0.40 ohm cm. The measured Hall mobility at microwave frequencies (with a size correction) was compared with the dc Hall mobility between 30°K and 300°K. The agreement was excellent.