  We discuss how lensing by magnetic fields in galaxy clusters affects
ultrahigh energy cosmic ray (UHECR) observations. As specific example, we use
Virgo together with the cluster magnetic fields obtained earlier in a
constrained simulation of structure formation including MHD processes. We find
that, if M87 is the single source of UHECRs from Virgo, the emitted flux is
strongly anisotropic in the most interesting energy range, (50-100)EeV, and
differs from the average value by a factor five or more for a significant
fraction of observers. Since magnetic lensing is energy dependent, the external
energy spectrum as seen by different observers varies strongly too. These
anisotropies are averaged out in the case that all active galactic nuclei in
Virgo emit UHECRs. In both cases, the anisotropies of the emitted UHECR flux
may introduce an important bias in the interpretation of UHECR data like, e.g.,
the determination of the source density n_s and the source energy spectrum of
UHECRs.
