Experimental studies of current instabilities in uniaxially compressed n- and p-type germanium are described. The instability reflects the presence of bulk negative differential conductivity due to transfer of carriers between the strain-split high-mobility and low-mobility states of the conduction or valence band. At low temperatures, threshold fields of the order of several hundred volts per centimeter are observed, which is an order of magnitude smaller than the threshold electric field for the Gunn effect in GaAs. In n-type Ge, the effect is observed at room temperature. The dependence of these phenomena on temperature, sample orientation, and stress magnitude is discussed. These results demonstrate the importance of intervalley transfer to the X minima in determining the high-field transport properties of n-type Ge. Unsuccessful attempts to observe similar effects in strained p-type InSb and n-type Si are also described and discussed. © 1969 The American Physical Society.