Stability of open universes

In this Comment I discuss the application of the dynamic systems approach to the stability analysis of relativistic open cosmological models. Although the method is powerful and in principle applicable to more realistic situations than those discussed here, I point out that an analysis recently carried out is flawed by a wrong assumption on the allowed range for a scale parameter q and by a misinterpretation of the perturbation variables. Perturbations in models with the equation of state p = wμ ( 0 < w = const ≤1) either have a sound wave character or asymptotically decay; for w = 0 (dust) all the perturbations asymptotically reach a constant value. A Laplace instability is found only for the unphysical values w > 1.