One of the electron paramagnetic resonance (EPR) spectra related to thermal donors in germanium (``spectrum 1'') has been studied in detail. The mutual identification of spectrum 1 and the F series of far-infrared (ir) donor energy levels has been established following observation of the effect of 100% bistability of that particular thermal-donor species. To our knowledge this is the first time that the bistability of the thermal-donor center has been observed in EPR. It has been further established that for the full description of spectrum 1 a higher spin value S=1 was necessary and the spin Hamiltonian had to be supplemented with an unusual higher-order term of the form ${\mathit{B}}^{2}$${\mathit{S}}^{2}$.