  Scalar perturbations can grow during a phantomic cosmological phase as the
big rip is approached, in spite of the high accelerated expansion regime, if
the equation of state is such that $\frac{p}{\rho} = \alpha < - {5/3}$. It is
shown that such result is independent of the spatial curvature. The perturbed
equations are exactly solved for any value of the curvature parameter $k$ and
of the equation of state parameter $\alpha$. Growing modes are found
asymptotically under the condition $\alpha < - {5/3}$. Since the Hubble radius
decreases in a phantom universe, such result indicates that a phantom scenario
may not survive longtime due to gravitational instability.
