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 -1leq 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.
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