EN
We give a characterization of the pairs of weights (v,w), with w in the class $A_∞$ of Muckenhoupt, for which the fractional maximal function is a bounded operator from $L^p(X,vdμ)$ to $L^q(X,wdμ)$ when 1 < p ≤ q < ∞ and X is a space of homogeneous type.