The removal of ammonia from fish hatchery effluents using nitrifying bacterial filters was demonstrated in pilot plant experiments. The filters used in the study showed maximum ammonia removal rates of from 150-200 milligrams of ammonia-nitrogen per hour per cubic foot of filter and the ammonia removal rates in the filters increased slightly as flow rates increased. The experiment also showed the effect of water recycle on water quality and the feasibility of using a 100% recycle system on a large scale at Alchesay National Fish Hatchery during periods when the regular water source is unsuitable for use due to high turbidity levels. Higher recycle rates caused lower ammonia concentrations under equilibrium conditions and the additional effects of filtration included removal of turbidity and a considerable consumption of oxygen in the reduction of carbonaceous BOD. The qualitative behavior of rainbow trout subjected to recycle conditions was observed and it was concluded that when filter size and aeration were adequate, fish hatchery effluent could be treated in nitrifying bacterial filters and recycled without any adverse effects on these fish. Mechanical difficulties prevented satisfactory operation of the large scale recycle experiment. (WRSIC abstract)