We compare the WMAP temperature power spectrum and SNIa data to models with a generalized Chaplygin gas as dark energy .
The generalized Chaplygin gas is a component with an exotic equation of state , p _ { X } = - A / \rho ^ { \alpha } _ { X } 
( a polytropic gas with negative constant and exponent ) .
Our main result is that , restricting to a flat universe and to adiabatic pressure perturbations for the generalized Chaplygin gas , the constraints at 95 % CL to the present equation of state w _ { X } = p _ { X } / \rho _ { X } 
and to the parameter \alpha are -1 \leq w _ { X } < -0.8 , 0 \leq \alpha < 0.2 , respectively .
Moreover , we show that a Chaplygin gas ( \alpha = 1 ) as a candidate for dark energy is ruled out by our analysis at more than the 99.99 \% CL .
A generalized Chaplygin gas as a unified dark matter candidate ( \Omega _ { CDM } = 0 ) 
appears much less likely than as a dark energy model , although its \chi ^ { 2 } is only two sigma away from the expected value .