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The electron spin resonance ͑ESR͒ of a two-dimensional electron system ͑2DES͒ in Al x Ga 1Ϫx As/GaAs in the regime of fractional filling ͑Ͻ1͒ of the lowest Landau level is investigated by a photoconductivity technique at millimeter wave frequencies. By performing the experiments in a standing wave the ESR is shown to be a magnetic-dipole transition. This is in agreement with a calculation based on an 8-band model and k•p theory. The ͑single-electron͒ theory also yields excellent agreement for the experimental and theoretical magnetic field dependences of the ESR transition energy, i.e., single-electron transition energies are measured and Kohn's theorem is not violated. Nevertheless, due to electron-electron interaction the ground state of the 2DES can be a many-particle state with similar amplitudes of spin-up and spin-down contributions. Evidence for such a reduced spin polarization is found in a striking reduction of the ESR strength when decreasing the temperature from 1.6 to 0.3 K.

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