Recently optically detected cyclotron resonance spectra reported by Ahmed et al. in ultra-pure n-GaAs have revealed low-energy structure associated with the cyclotron resonance. The authors have investigated the far-infrared photoconductive response under conditions of cyclotron resonance in these n-GaAs ( mu approximately=400000 cm² V^-1 s^-1) samples and also in n-InP ( mu approximately=170000 cm² V^-1 s^-1) samples grown by MBE. The suggestion that the structure arises from transitions involving non-parabolicity and polaron coupling effects is discounted using a five-level P.p calculation as found in the literature. Other possible origins, supported by appropriate theory, are suggested for this structure.