The fatigue phenomena of vacuum and gas‐filled cesium‐antimony photocathodes were studied. It is shown that the phototubes can be operated without fatigue at high photocurrent densities, about 90 &mgr;a cm−2, by reducing the temperature rise of the cathode due to high illumination and by lowering the anode voltage to be applied.The photocathodes whose responses ranged from 30 to 60 &mgr;a/lu were used to study the changes of their sensitivities in the following experiments: (a) Effect of intense illumination on the sensitivity; (b) variation in the response by heating at elevated temperatures and (c) relation between the change in photoresponse and the positive potential applied to the anode. The results obtained were as follows: In red radiation the thermal effects dominated, and in blue‐green radiation an effect on the photosensitivity due to the absorption in the cesium anti‐monide film appears. The photoresponse was appreciably reduced when a positive potential above the ionization potential of the gas in the phototubes was applied to the anode. At a very high light level the fatigue phenomena of the cathodes may be caused mainly by the thermal decomposition of cesium antimonides.