An E.S.R. study of amorphous silicon films evaporated under U.H.V. conditions is presented. The question of whether the ESR signal arises from randomly distributed individual spins or spin clusters is investigated using several experimental tests : a) Dependence of the line‐width on spin concentration, b) Temperature dependence of the susceptibility from 5 to 120 °K, c) Influence of evaporating at oblique incidence. Present results are compatible with a random distribution of individual spins 1/2.