  We derive an analytical expression for the growth rate of matter density
perturbations on the phantom brane (which is the normal branch of the
Dvali-Gabadadze-Porrati model). This model is characterized by a phantomlike
effective equation of state for dark energy at the present epoch. It agrees
very well with observations. We demonstrate that the traditional
parametrization $f=\Omega_m^\gamma$ with a quasiconstant growth index $\gamma$
is not successful in this case. Based on a power series expansion at large
redshifts, we propose a different parametrization for this model:
$f=\Omega_m^\gamma\left(1+\frac{b}{\ell H}\right)^\beta$, where $\beta$ and $b$
are constants. Our numerical simulations demonstrate that this new
parametrization describes the growth rate with great accuracy - the maximum
error being $\leq 0.1\%$ for parameter values consistent with observations.
