  Ultrahigh energy protons in magnetic fields produce pions and thus lose
energy. The mean free path of such a process is worked out for Gaussian random
fields. Two cases are considered: an isotropic and a cylindrically symmetric
distribution. The energy loss is proportional to E^3 <B^2>; it becomes
significant for protons of energies larger than about 10^19 eV and magnetic
fields of about 10^9 Gauss. For energies and magnetic fields of this magnitude,
a proton injected into the magnetic field loses a substantial fraction of its
initial energy due to pion production.
