The distinguishing index D ′ (Γ) of a graph Γ is the least number k such that Γ has an edge-coloring with k colors preserved only by the trivial automorphism. In this paper we prove that if the automorphism group of a finite graph Γ is simple, then its distinguishing index D ′ (Γ) = 2.The distinguishing index of a graph Γ has been introduced by Pilśniak and Kalinowski [9] in 2015 to be the least number d such that Γ has an edge-coloring with d colors breaking the symmetry of Γ (i.e., such that no nontrivial automorphism of Γ preserves this coloring). This is an analog to the notion of the distinguishing number D(Γ) of a graph introduced by Albertson and Collins [1] in 1996, which has been defined in the same way for colorings of vertices.Note that for asymmetric graphs we have D(Γ) = D ′ (Γ) = 1. For other graphs D(Γ) ≥ 2, and it is conjectublack that almost all of them have the distinguishing number two (see [4,9]). The situation is similar for the distinguishing index, and the claim that having the distinguishing index two is generic for asymmetric graphs has been supported by results in [10,11,14].The concepts of the distinguished number and distinguished index generalize naturally to the distinguishing number of an arbitrary group action ([15, 3]). Following this generalization, it was realized in [2] that in permutation group theory the problem had been investigated for many years as a part of the study of set stabilisers of group actions, and some results may be successfully applied. In particular, a result by Gluck [6] (obtained as early as in 1983) shows that if the order of the automorphism group of a graph Γ is odd (and > 1), then both the distinguishing number and the distinguishing index of Γ are two,This paper is the sequel of [8], where we have proven, in particular, that if the automorphism group of a finite graph Γ is simple, then its distinguishing number D(Γ) = 2. To obtain this result we have described the distinguishing number for all possible actions of simple groups. Now, we apply the latter to show that for such graphs also the distinguishing index D ′ (Γ) = 2. This is not so straightforward as in the case of Gluck's result. It requires to consider