  The generalized Chaplygin gas model is characterized by the equation of state
$p = - \frac{A}{\rho^\alpha}$. It is generally stated that the case $\alpha =
0$ is equivalent to a model with cosmological constant and dust ($\Lambda
CDM$). In this work we show that, if this is true for the background equations,
this is not true for the perturbation equations. Hence, the mass spectrum
predicted for both models may differ.
