The electron drift‐velocity–electric‐field relationship has been calculated for the Ga1−xInxP1−yAsyquaternary alloy using the Monte Carlo method. Emphasis has been placed on the compositional range for which the alloy is lattice matched to GaAs and InP. These calculations suggest that this quaternary offers promise as a material for microwave semiconductor devices, including field‐effect transistors and transferred electron devices.