It has been found that the transverse magnetic field drastically reduces the current gain of an AlGaAs/GaAs hot-electron transistor (HET) at 4.2 K. The result can be understood in terms of the cyclotron motion of hot electrons within the base layer (1000 Å thick). Estimated subpicosecond base transit time confirms the quasiballistic transport of hot electrons across the base layer of the HET.