| Summary: | This work generalises earlier works on the growth of the density perturbations in Bianchi type I cosmologies filled with dust to include also the effects of pressure anda positive cosmological constant. For the analysis the 1+3 covariant split of space-time formalism is used. As the perturbative variables we use scalar quantities thatare zero on the background, and hence are gauge-invariant. These variables form acoupled closed system of first-order evolution equations, that is analysed numericallyand analytically. For short wavelengths an oscillatory behavior is obtained, whereasfor long wavelengths the energy density perturbations grow unboundedly.
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