We consider the problem of finding the optimal exponent in the Moser-Trudinger inequalityHere Ω is a bounded domain of R N (N ≥ 2), s ∈ (0, 1), sp = N , W s,p 0 (Ω) is a Sobolev-Slobodeckij space, and [·] W s,p (R N ) is the associated Gagliardo seminorm. We exhibit an explicit exponent α * s,N > 0, which does not depend on Ω, such that the Moser-Trudinger inequality does not hold true for α ∈ (α * s,N , +∞).