The photothermal spectrum of shallow donors in n-GaAs has been investigated at various magnetic field strengths up to 4T at different temperatures between 1.4 and 12K. Weak spectral structures observed at frequencies above the dominant ls --~ 2p+1 line are attributed to optical excitations into metastable states which arise in a magnetic field. The final states of these transitions can be unambiguously assigned by using high field quantum numbers.The optical excitation spectra of hydrogen-like shallow donors in high purity GaAs epitaxial layers have been studied extensively in the past by photothermal ionisation spectroscopy. The techniques of high resolution far-infrared Fourier spectroscopy [1,2,3] as well as far-infrared laser magnetospectroscopy [4] have been applied yielding detailed knowledge of donor levels in a wide range of magnetic field strengths. Since the early work of Larsen [5] many variational calculations based on the effective mass approximation have been performed reproducing the bound donor levels in excellent agreement with experimental results [6].The most prominent lines observed in photoconductivity spectra at low temperatures correspond to transitions from the ls donor ground state to 2p:1:1 states where nlm is the low field hydrogen atom energy level notation. Besides these strong lines, weak transitions like ls --~ 2s are observed violating the electric dipole selection rules. This effect is attributed to admixing of p-states due to the Stark effect caused by the strong electric fields of ionized impurities in the partially compensated material. The photoconductivity spectra of n-GaAs in a magnetic field contain, however, additional weak structures which have not yet been identified. In the present study we show that most of these lines, being observed in Faraday configuration, may be grouped in pairs with frequency separation ~:c, where v:c = eB/m* is the conduction band cyclotron frequency. Taking into account electric dipole selection rules Am : ±1 and 7riTrf : -1, where 7ri,j" are the parities of the initial and final states respectively, we conclude that most of the observed structures are due to excitation of metastable states from the donor ground state. Metastable states arise in a magnetic field out of the donor continuum and do not lead to donor bound states in the zero field limit. The final states may be assigned by high field quantum numbers (N,m,t¢) where N and m denote the Landau levels and the angular momentum respectively and counts the donor states below each (N, m) Landau subband. The parity of a state in the high field notation is given by ~r = (-1)m+'L The measurements were carried out on an n-GaAs epitaxial layer of 41/~m thickness predominantly containing silicon shallow donors with an effective concentration N D -NA = 9.2 • 1013 cm -3, The mobility at liquid nitrogen temperature was/z = 9.4.104 cm2/Vs. Ohmic point contacts were alloyed at opposite edges of the layer and a wedged piece of polished semiinsulating GaAs was glued with paraffin oil onto the substrate...