We establish the existence of smooth stable manifolds for
nonautonomous differential equations $v'=A(t)v+f(t,v)$ in a Banach
space, obtained from sufficiently small perturbations of a linear
equation $v'=A(t)v$ admitting a nonuniform exponential
dichotomy. In addition to the exponential decay of the flow on the
stable manifold we also obtain the exponential decay of its
derivative with respect to the initial condition. Furthermore, we
give a characterization of the stable manifold in terms of the
exponential growth rate of the solutions.