A formula is derived for the average rate of energy loss suffered by a charged particle moving in a helical orbit in a homogeneous medium. In a cold, magnetized plasma it is shown that the Cˇerenkov excitation of the whistler and ion cyclotron waves depends critically on certain characteristic wave speeds relative to the component of the particle's velocity parallel to the magnetic field. The power spectrum of Cˇerenkov radiation exhibits sharp peaks not only at the frequencies of the lower and upper hybrid resonances but also at those frequencies at which the ray phase velocity equals the particle's velocity,v∥, parallel to the magnetic field. An intriguing feature of this radiation is that the more slowly the particle moves along the magnetic field, the greater is the emitted radiation; the power emitted in the whistler mode also becomes large whenv∥equals the group speed of those waves that are beamed along the magnetic field. By using a simple graphical construction one can readily locate those directions in which the angular distribution of synchrotron radiation is sharply peaked.