  Ultra high energy neutrinos are produced by the interaction of hadronic
cosmic rays with the cosmic radiation background. More exotic scenarios like
topological defects or new hadrons predict even larger fluxes. In particular,
Earth-skimming tau neutrinos could be detected by the Fluorescence Detector
(FD) of Pierre Auger Observatory. A detailed evaluation of the expected number
of events has been performed for a wide class of neutrino flux models. An
updated computation of the neutrino-nucleon cross section and of the tau energy
losses has been carried out. For the most optimistic theoretical models, about
one Earth-skimming neutrino event is expected in several years at FD.
