  We study the impact of a generalized Chaplygin gas as a candidate for dark
energy on CMB anisotropies. The generalized Chaplygin gas is a fluid component
with an exotic equation of state, $p = - A/\rho^\alpha$ (a polytropic gas with
negative constant and exponent). Such component interpolates in time between
dust and a cosmological constant, with an intermediate behaviour as $p=\alpha
\rho$. Perturbations of this fluid are stable on small scales, but behave in a
very different way with respect to standard quintessence. Moreover, a
generalized Chaplygin gas could also represent an archetypal example of a
phenomenological unified models of dark energy and dark matter. The results
presented here show how the CMB anisotropies induced by this class of models
differ from a $\Lambda$CDM model.
