  Starting with Ihara's work in 1968, there has been a growing interest in the
study of zeta functions of finite graphs, by Sunada, Hashimoto, Bass, Stark and
Terras, Mizuno and Sato, to name just a few authors. Then, Clair and
Mokhtari-Sharghi have studied zeta functions for infinite graphs acted upon by
a discrete group of automorphisms. The main formula in all these treatments
establishes a connection between the zeta function, originally defined as an
infinite product, and the Laplacian of the graph. In this article, we consider
a different class of infinite graphs. They are fractal graphs, i.e. they enjoy
a self-similarity property. We define a zeta function for these graphs and,
using the machinery of operator algebras, we prove a determinant formula, which
relates the zeta function with the Laplacian of the graph. We also prove
functional equations, and a formula which allows approximation of the zeta
function by the zeta functions of finite subgraphs.
