We use a special version of Runge's method to prove that there exists an integer p_0 such that for any non-CM elliptic curve E over Q and any prime p>p_0 the image of the representation of the absolute Galois group of Q induced by the Galois action on the p-division points of E is not contained in the normalizer of a split Cartan subgroup. This gives a partial answer to an old question of Serre. A joint work with P. Parent.