  Recent work suggests that the cosmic ray spectrum may be dominated by
Galactic sources up to ~10^{17.5} eV, and by an extra-Galactic component
beyond, provided this latter cuts off below the transition energy. Here it is
shown that this cut-off could be interpreted in this framework as a signature
of extra-galactic magnetic fields with equivalent average strength B and
coherence length l_c such that B\sqrt{l_c} ~ 2-3.10^{-10} G.Mpc^{1/2}, assuming
l_c < r_L (Larmor radius at 10^{17} eV) and continuously emitting sources with
density 10^{-5}/Mpc^3. The extra-Galactic flux is suppressed below 10^{17} eV
as the diffusive propagation time from the source to the detector becomes
larger than the age of the Universe.
