  We present an analytical fit to the growth function of the dark matter
perturbations when dark energy perturbations are present. The growth index
depends upon the dark energy equation of state w, the speed of sound of the
dark energy fluctuations, the dark matter abundance and the observed comoving
scale. The growth index changes by ~5% for small speed of sound and large
deviations of w from -1 with respect to its value in the limit of no dark
energy perturbations.
