  We consider the electron-positron plasma generation processes in the
magnetospheres of magnetars - neutron stars with strong surface magnetic
fields, B = 10^(14) - 10^(15) G. We show that the photon splitting in a
magnetic field, which is effective at large field strengths, does not lead to
the suppression of plasma multiplication, but manifests itself in a high
polarization of gamma-ray photons. A high magnetic field strength does not give
rise to the second generation of particles produced by synchrotron photons.
However, the density of the first-generation particles produced by curvature
photons in the magnetospheres of magnetars can exceed the density of the same
particles in the magnetospheres of ordinary radio pulsars. The plasma
generation inefficiency can be attributed only to slow magnetar rotation, which
causes the energy range of the produced particles to narrow. We have found a
boundary in the P - Pdot diagram that defines the plasma generation threshold
in a magnetar magnetosphere.
