Suppose M is a
closed orientable 3-manifold. Then H. Hilden et al. proved that M is a 3-fold
branched covering of S3 branched over a fibered knot. In this paper we prove
that, if M is irreducible and atoroidal, then M is either a 3-fold branched
covering of S3 branched over a simple, fibered knot, or a 2-fold branched
covering of a closed orientable 3-manifold whose Heegaard genus is at most
one.