The ferromagnetic semiconductor (Ga,Mn) As has emerged as the most studied material for prototype applications in semiconductor spintronics. Because ferromagnetism in (Ga, Mn) As is hole-mediated, the nature of the hole states has direct and crucial bearing on its Curie temperature T-C. It is vigorously debated, however, whether holes in (Ga, Mn) As reside in the valence band or in an impurity band. Here we combine results of channelling experiments, which measure the concentrations both of Mn ions and of holes relevant to the ferromagnetic order, with magnetization, transport, and magneto-optical data to address this issue. Taken together, these measurements provide strong evidence that it is the location of the Fermi level within the impurity band that determines T-C through determining the degree of hole localization. This finding differs drastically from the often accepted view that T-C is controlled by valence band holes, thus opening new avenues for achieving higher values of T-C.