Abstract. Given Γ ⊂ Q * a multiplicative subgroup and m ∈ N + , assuming the Generalized Riemann Hypothesis, we determine an asymptotic formula for the number of primes p ≤ x for which the indp Γ = m, where indp Γ = (p − 1)/|Γp| and Γp is the reduction of Γ modulo p. This problem is a generalization of some earlier works by Cangelmi-Pappalardi, Lenstra, Moree, Murata, Wagstaff and probably others. We prove, on GRH, that the primes with this property have a density and, in the case when Γ contains only positive numbers, we give an explicit expression for it in terms of an Euler product. We conclude with some numerical computations.Mathematics Subject Classification (2010). Primary 11R45; Secondary 11A07.