A technique is described for time resolving, to nanosecond precision, the velocity distribution of a periodically varying electron beam and for generating a direct plot of the entire phase plane distribution function. Calculations are presented to indicate the maximum time rate‐of‐change of the distribution function which may be accurately observed with this technique. The high‐frequency response obtainable with this technique makes it a powerful diagnostic tool for use in studying the interaction of an electron beam with plasmas and, in particular, the trapping phenomenon in beam‐plasma interactions.