A model, based on energy-flow considerations, is presented which describes the population dynamics of Brachionus rubens in the second stage of a two-stage algalrotifer chemostat. The rotifers are foodlimited with substrate-inhibition occurring at high algal densities. The model shows two stable states: steady state with constant density of rotifers and washout of the animals. Which one of the stable states is reached depends on the initial conditions.Empirical data are in general agreement with the model. Deviations may be explained by the fact that the data underlying the model calculations are based on a different food alga (Chlorella vulgaris) than the one used in the experiments (Monoraphidium minutum).The observed population growth rate reaches a maximum value of 0.84 (1/day) at algal densities of 3-4. 106 cells/ml. It decreases at higher algal densities. The egg ratio is related linearily to algal density without being reduced at high algal densities.