Measurements of the counting rates (∼103to 104counts/sec) of continuous‐channel electron multipliers mounted in an electrostatic analyzer responding to a monoenergetic beam of electrons, while operating in a vacuum chamber at a pressure ∼3×10−6Torr attained with an oil diffusion pump, display a degradation of their gain (fatigue) which is proportional to the accumulated counts. The useful lifetime of these devices when employed with fixed‐threshold pulse amplifiers is defined here as the accumulated counts until gain degradation has produced a reduction of the counting rates to 15% of the initial responses at an operating bias voltage of 4000 V and constant stimuli. The lifetimes of these particle detectors in this laboratory environment are ∼1010counts or, for example, an average counting rate of 300 counts/sec for one year. Comparison of this laboratory lifetime with the responses of similar instrumentation which has been flown on the earth satellites OGO's 3, 4, and 5 and IMP 4 demonstrates that the expected lifetimes for these electron multipliers in a spaceflight environment are several years. Efficiencies of an electron multiplier for counting monoenergetic electrons over an energy range ∼60 to 50 000 eV are also presented.