Summary: A ratio-dependent chemostat model with production of toxin is investigated. A detailed qualitative analysis about existence of its solutions and the local asymptotic stability of its equilibria is carried out. Using the Lyapunov-LaSalle invariant principle, we obtain sufficient conditions for the global asymptotic stability of the equilibria. Our results demonstrate that under different conditions, not only the principle of competitive exclusion but also the coexistence of competition is established.