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This paper focuses on the analysis of the gravitational instability inpresence of bulk viscosity both in Newtonian regime and in thefully-relativistic approach. The standard Jeans Mechanism and theQuasi-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 isfounded to be damped by viscosity and the top-down mechanism of structurefragmentation is suppressed. The value of the Jeans Mass remains unchanged alsoin presence of viscosity. In the relativistic approach, we get a power-lawsolution existing only in correspondence to a restricted domain of $\zo$.

Gravitational Instability in Presence of Bulk Viscosity: the Jeans Mass and the Quasi-Isotropic Solution