  We prove an equivariant Grothendieck-Ogg-Shafarevich formula. This formula
may be viewed as an \'etale analogue of well-known formulas for Zariski sheaves
generalizing the classical Chevalley-Weil formula. We give a new approach to
those formulas (first proved by Ellingsrud/L{\o}nsted, Nakajima, Kani and Ksir)
which can also be applied in the \'etale case.
