Fourier transform infrared photoconductivity spectra of Si donors in GaAs under magnetic field are presented in this paper. Four new spectral lines corresponding to the transitions from hydrogen-like ground state to the metastable states (510), (610), (710), and (810) under nonzero magnetic field were observed. The variational calculation model proposed by Barmby &etal; was applied to the high-lying metastable states. The experimental data of the transition energies agree well with the variational results under high magnetic field and below the resonant polaron region. Deviations due to omitting the bound Landau continuum coupling in the trial wave function were found in the comparison at the low field region for the metastable states with higher Landau index. The ionization energies of the high-lying metastable states were also determined. ©1997 American Institute of Physics.