  The spectrum of cyclotron radiation produced by electrons with a strongly
anisotropic velocity distribution is calculated taking into account higher
harmonics. The motion of the electrons is assumed to be ultrarelativistic along
the magnetic field and nonrelativistic across the field. One characteristic
feature of the resulting spectrum is that harmonics of various orders are not
equally spaced. The physical properties and observed spectra of four X-ray
pulsars displaying higher cyclotron harmonics are analyzed. The cyclotron
features in the spectra of all four pulsars can be interpreted only as emission
lines. Moreover, the observed harmonics are not equidistant, and display
certain other properties characteristic of emission by strongly anisotropic
ultrarelativistic electrons. In addition, there are indirect theoretical
arguments that the electrons giving rise to cyclotron features in the spectra
of X-ray pulsars are ultrarelativistic and characterized by strongly
anisotropic distributions. As a result, estimates of the magnetic fields of
X-ray pulsars (which are usually derived from the energies of cyclotron lines)
and certain other physical parameters require substantial revision.
