Abstract. Given a faithful action of a finite group G on an algebraic curve X of genus g X ≥ 2, we give explicit criteria for the induced action of G on the Riemann-Roch space H 0 (X, O X (D)) to be faithful, where D is a G-invariant divisor on X of degree at least 2g X − 2. This leads to a concise answer to the question when the action of G on the space H 0 (X, Ω ⊗m X ) of global holomorphic polydifferentials of order m is faithful. If X is hyperelliptic, we furthermore provide an explicit basis of H 0 (X, Ω ⊗m X ). Finally, we give applications in deformation theory and in coding theory and we discuss the analogous problem for the action of G on the first homology H 1 (X, Z/mZ) if X is a Riemann surface.