The cosmic ray spectrum has been shown to extend well beyond 10 20 eV. With nearly 20 events observed in the last 40 years, it is now established that particles are accelerated or produced in the universe with energies near 10 21 eV at the production site. In all production models neutrinos and photons are part of the cosmic ray flux. In acceleration models (bottom-up models), they are produced as secondaries of the possible interactions of the accelerated charged particle; in direct production models (top-down models) they are a dominant fraction of the decay chain. In addition, hadrons above the GZK threshold energy will also produce, along their path in the Universe, neutrinos and photons as secondaries of the pion photo-production processes. Therefore, photons and neutrinos are very distinctive signatures of the nature and distribution of the potential sources of ultrahigh energy cosmic rays. In the following we describe the tau neutrino detection and identification capabilities of the Auger Observatory. We show that in the range 3×10 17–3×10 20 eV the Auger effective aperture reaches a few tenths of km 2 sr, making the Observatory sensitive to fluxes as low as a few tau neutrinos per km 2 sr year. In the hypothesis of ν μ→ν τ oscillations with full mixing, this sensitivity allows to a probe of the GZK cutoff as well as providing model independent constraints on the mechanisms of production of ultrahigh energy cosmic rays.