  This paper focuses on the analysis of the gravitational instability in
presence of bulk viscosity both in Newtonian regime and in the
fully-relativistic approach. The standard Jeans Mechanism and the
Quasi-Isotropic Solution are treated expressing the bulk-viscosity coefficient
$\zeta$ as a power-law of the fluid energy density $\rho$, i.e.,
$\zeta=\zo\rho^{s}$. In the Newtonian regime, the perturbation evolution is
founded to be damped by viscosity and the top-down mechanism of structure
fragmentation is suppressed. The value of the Jeans Mass remains unchanged also
in presence of viscosity. In the relativistic approach, we get a power-law
solution existing only in correspondence to a restricted domain of $\zo$.
