In the presence of an ambient medium and an external magnetic field, ultrarelativistic charged particles emit synchrotron‐?erenkov radiation. For cases in which the classical synchroton frequency greatly exceeds any characteristic absorption frequency of the medium, i.e., when the medium can effectively be treated as a free‐electron gas, the photon production rates depend sensitively on the ratio of the synchroton frequency to the relativistically boosted electron plasma frequency. In principle this connection can be exploited for the construction of high‐energy electron detectors (E≳2 GeV). These counters allow for the simultaneous determination of the electron energy, and a mass discrimination between electrons and higher‐mass singly charged particles. The relative merits of synchrotron‐?erenkov detectors and some standard counters are briefly discussed.