The fundamental absorption lines of substitutional boron acceptor impurity in germanium, which lie between 1 and 3 THz, now have been measured in magnetic fields to 18 T, greatly extending the reach of both previous experiments (to 7 T)and theory (to 10 T). The Faraday configuration was employed with the magnetic field B||. Unexpected behavior has been observed relating to the magnetic-field-induced splitting of the ground and first two excited states (all of which are fourfold degenerate): (a)One pair of Zeeman ground states splits at only half the rate predicted with field; this behavior continues to high field. The other pair shows a rapid increase with field. (b) The two pairs of Zeeman states emerging from the first excited state initially separate with field, then, above 10 T, converge, almost meeting by 18 T. (c)One pair of the Zeeman states from the second excited state begins to plateau at high field; the other shows a dramatic decrease above 9 T, extrapolated to become zero at around 24 T. Taken together, these results suggest modification to the existing theory is required, and may have implications for quantum computation involving substitutional impurities.