  We consider the problem of how to construct the curvature perturbation
$\zeta$ to nonlinear levels, which is expected to evolve time independently on
super-horizon scales; in particular we concentrate on the situation where the
universe is dominated by a perfect fluid. We have used a low energy/long
wavelength effective action to model the fluid sector. Different from previous
work, our approach assumes neither the absence of vector and tensor
perturbations nor ``local homogeneity and isotropy''. As a corollary, we also
show that the nonlinearly defined graviton field $\gamma_{ij}$ is conserved
outside the horizon in the same manner as $\zeta$ is.
