The hypocentral parameters to t0, φ0, λ0, h0, for the main shock occurred in Friuli on May 6th 1976 and itsfore shock were determined by the first arrival method. The computation was tried employing travel-time curves for different crustal models. The results show a very low influence of the travel-time curves on epicentral coordinates, whose error is essentially dependent on the precision of station data. The focal depth, on the contrary, is a parameter directly connected to the model used in the computation, so, if the model is not accurately known, the indetermination can be of the same order of magnitude as the crust thickness, even using very precise recorded data.