  We find a new instability in the four-dimensional Reissner-Nordstrom-de
Sitter black holes against charged scalar perturbations with vanishing angular
momentum, $l=0$. We show that such an instability is caused by superradiance.
The instability does not occur for an larger angular index, as explicitely
proven for $l=1$. Our results are obtained from a numerical investigation of
the time domain profiles of the perturbations.
