The fast neutron irradiation behavior of germanium and silicon Esaki diodes has been experimentally examined. The dominant change produced is an increase in the ``excess'' current which is proportional to integrated neutron flux. The observed increase in the vicinity of the current minimum is approximately 2.6×10−15amp/fast neutron and 1.1×10−14amp/fast neutron for germanium and silicon diodes respectively. Substantial changes result in the voltage‐current characteristics of the diodes employed in the decade of exposure between 1016–1017fast neutrons/cm2for germanium diodes and between 1015–1016fast neutrons/cm2for silicon diodes. One kilomegacycle cavity oscillators employing germanium diodes exhibit a marked reduction in power output in the decade of exposure between 1016–1017fast neutrons/cm2. The magnitude of the decrease is in approximate agreement with the observed bombardment reduction of diode negative conductance.