We report on measurements of the electron spin resonance (ESR) signal from amorphous Si (a‐Si) with spin concentrations as low as 3×1016cm−3. The a‐Si was prepared by the glow discharge decomposition of silane gas. From a study of the linewidth, lineshape, saturation and spin susceptibility and from their variation with temperature we present evidence that even for samples with &angupr;1016spins cm−3, there are some spins that undergo an antiferromagnetic spin exchange interaction. The Curie‐Weiss &Vthgr; in the relation &khgr; =C/(T+&Vthgr;) is found to be 1.3±0.4 K. We find that &Vthgr; is independent of spin concentration in our samples and within experimental uncertainties agrees with the &Vthgr; reported for evaporated and sputtered films with as many as 8×1019spins cm−3. The constancy of &Vthgr; in samples differing by more than three orders of magnitude in spin concentration implies the existence of spin clusters in a‐Si.