An approach to the modelling of suspended-growth anaerobic digestion systems based on the assumption of an incompletely mixed reactor is presented. The mathematical model developed describes the dynamic behaviour of anaerobic sludge digesters under non-ideal mixing conditions. The microbial kinetic model for the anaerobic digestion of waste-activated sludge distinguishes the processes of death and lysis of activated sludge cells, hydrolysis of particulate material, fermentation of soluble substrates, volatile fatty acids utilisation and methane formation. The interaction of two microbial groups is considered, i.e. acid-formers and methanogens. Their growth is assumed to depend on Monod kinetics for the substrates. Death and lysis, hydrolysis and biomass decay are described by first order reactions. The biokinetic expressions were linked to a simple mixing model which considered the reactor volume split into two sections : the flow-through and the retention regions. The transfer of material between regions was assumed to be limited. Deviations from an ideal completely mixed regime were represented by changing the relative volume of the flow-through region (alpha) and the turnover time of material in the vessel (tau). The dynamic model described the effects of the retention time and reactants＇ distribution, resulting from the mixing condition, on process performance. Computer simulations under different conditions showed a considerable decline in methane production and treatment efficiency due to incomplete mixing. The COD removal efficiency increased by extending the retention time and the degree of mixing. The evaluation of the impact of the mixing parameters showed that alpha has a far more significant effect on the performance of anaerobic digestion than tau does. Nevertheless, both are important and the overall efficiency is a complex function of both parameters. The results obtained confirm and emphasise the importance of considering mixing when simulating anaerobic digestion, calculating process conversion efficiency, and during anaerobic reactor design.