  Zitterbewegung of neutral relativistic particles propagating along a constant
magnetic and/or electric field is studied. It is shown that spin
Zitterbewegung, when superimposed on the Larmor precession frequency, leads to
a beating pattern. The existence of a forbidden frequency of spin precession is
predicted. Modifications of position and velocity Zitterbewegung due to lifted
spin degeneracy manifested in the appearance of longitudinal and transversal
Zitterbewegung, each with two Zitterbewegung frequencies and resulting beating
patterns, are reported.
