The influence on magnetic field on l/fnoise in a planar GaAs resistor grown by molecularbeam epitaxy and without surface effects was investigated experimentally. The experimental results can be explained by the number fluctuation model but not by the mobility fluctuation modeL Previously, experimental results indicating number fluctuation type of l/fnoise were mostly attributed to the surface effects associated with the particular structures used for the experiments. In our device the surface effects were diminished so that the fluctuations of the bulk current could be considered to produce the 1/fnoise.