Summary: 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 are \(-1 \leq w_X < -0.8, 0 \leq \alpha < 0.2\), respectively. Moreover, we show that a Chaplygin gas \((\alpha = 1)\) is ruled out as a candidate for dark energy by our analysis at more than the 99.99% CL. A generalized Chaplygin gas appears much less likely as a unified dark matter candidate \((\Omega_{CDM} = 0)\) than as a dark energy model, although its \(\chi^2\) is only two sigma away from the expected value.