We present the results of measurements of the noise temperature of a heterostructure field-effect transistor (HFET) low-noise amplifier operating at 683 MHz in high static magnetic fields. These measurements indicate that the amplifier noise temperature increases from 4.2 K in zero field to 7.0 K at 3.6 T. The effect is highly dependent on the orientation of the amplifier with respect to the magnetic field. We propose an explanation of this effect in which the paths of the electrons in the HFET channel are altered due to the Lorentz force from the applied field, affecting the HFET transconductance. Calculations based on this model are shown to be consistent with our measurements. ©1997 American Institute of Physics.