Experiments for the study of denitrification of biologically stabilised, nitrified pig waste are described. The rate of denitrification was increased by the addition of an energy source or by an increase in temperature. A supplementary energy source in field-scale stabilisation systems can be replaced by the residual chemical oxygen demand of a partially nitrified waste. A semi-continuous denitrification process was capable of reducing input inorganic N concentrations of several hundred mgl-1 to less than 50 mgl-1. Denitrification also improved final effluent quality by reducing soluble phosphorus, total and dissolved solids and chemical oxygen demand.