By modulating a beam strongly with a high‐level signal, called the ``pump'' signal, a low‐level space charge wave of another frequency imposed on the beam can be parametrically amplified. With the pump frequency twice the signal frequency, 13‐db gain per plasma wavelength has been observed in a movable cavity experiment. Over a 10‐in. length of beam the signal has been observed to increase 41 db. When the imposed signal wave's frequency is not exactly half the pump frequency, the beam generates another wave called the ``idler'' such that &ohgr;signal+&ohgr;idler=&ohgr;pumpwith both signal and idler growing exponentially with distance along the beam. In another experiment with the pump frequency lower than the signal frequency, the signal was observed to increase by 30 db over a 9.2 in. length of beam.