The authors report on temperature-dependent Hall effect measurements on Si delta-doped GaAs samples grown by MBE at 480 °C, 530 °C and 620 °C. In the best sample grown at 480 °C the mobility is 6760 cm2 V-1 s-1 at 4.2 K. To the authors' knowledge this is the highest mobility ever reported in a delta-doped structure. From subband population measurements the spreading of the donors in the samples grown at low temperature is determined to be 20 Å. On these high-mobility samples they were able to perform the first reported cyclotron resonance measurements. The electron effective mass is found to be considerably higher than that at the Gamma-conduction band minimum in GaAs.