The effect of the mean microorganism residence time in the aeration vessel of a continuous culture system designedfor the treatment of piggery slurry was studied. Experiments using laboratory-scale reactors were repeated at a number of residence times within the range 0·5 to 16 days. The composition of treated slurry when each experiment reached steady state was measured and the analytical data correlated with mathematical models. Equations are presented to define the effect of residence time on the quality of treated slurry, in terms of COD, BOD, organic nitrogen, TS and TSS and the dissolved oxygen requirement of the biomassin the reactor. These equations provide a basis for the development of farm scale systems for the conversion of slurries of animal waste into more desirable products.