  We study numerically the interplay of disorder and attractive interactions
for spin-1/2 fermions in the three-dimensional Hubbard model. The results
obtained by projector quantum Monte Carlo simulations show that at moderate
disorder, increasing the attractive interaction leads to a transition from
delocalized superconducting states to the insulating phase of localized pairs.
This transition takes place well within the metallic phase of the
single-particle Anderson model.
