  We discuss as a new signature for the interaction of extragalactic ultrahigh
energy protons with cosmic microwave background radiation a spectral feature
located at $E=6.3\times 10^{19}$ eV in the form of a narrow and shallow dip. It
is produced by the interference of $e^+e^-$-pair and pion production. We show
that this dip and in particular its position are almost model-independent. Its
observation by future ultrahigh energy cosmic ray detectors may give the
conclusive confirmation that an observed steepening of the spectrum is caused
by the Greisen-Zatsepin-Kuzmin effect.
