This paper deals with the existence and the asymptotic behavior of non-negative solutions for a class of stationary Kirchhoff problems driven by a fractional integro-differential operator L-K and involving a critical nonlinearity. In particular, we consider the problem -M(parallel to u parallel to(2))L(K)u = lambda f(x, u) + vertical bar u vertical bar(2s*-2) u in Omega, u = 0 in R-n \ Omega, where Omega subset of R-n is a bounded domain, 2(s)* is the critical exponent of the fractional Sobolev space H-s(R-n), the function f is a subcritical term and lambda is a positive parameter. The main feature, as well as the main difficulty, of the analysis is the fact that the Kirchhoff function M could be zero at zero, that is the problem is degenerate. The adopted techniques are variational and the main theorems extend in several directions previous results recently appeared in the literature Ver menos