We report detailed two‐dimensionalinsitumeasurements of electron density inside an injected‐beam crossed‐field amplifier operating at 150 MHz. It is demonstrated that during amplification there is a spatial redistribution of the cycloidal electron beam as electrons are drawn to the anode. These measurements have been compared with a numerical simulation [G. E. Dombrowski, IEEE Trans. Electron. Devices35, 2060 (1988)] which predicts a change in electron trajectories but gives a larger electron loss than was measured.