The operating characteristics of laboratory waste treatment systems were studied during the aerobic degradation of pig excrement at different loading rates and temperatures. The treatment systems were of two types: one was operated with floc formation and gravity separation of liquid and suspended solid effluents; and a second was operated without floc formation or separation of the effluent into liquid and solid fractions. With an operating temperature of 15°C the parameters most affected by loading rate were (1) the concentrations of suspended solids and chemical oxygen demand in the liquid effluent; (2) the pH value of the mixed liquor; (3) nitrification; (4) the BOD of the supernatant from the mixed liquor; and (5) output of suspended solids as a percentage of input. The concentrations of suspended solids and chemical oxygen demand in the liquid effluents were little affected by loading rates in the range 0·05-0·15 g SS g MLSS-1 d-1 (0·02-0·06 g BOD g MLSS-1 d-1) but increased with increasing loading rate in the range 0·15-0·30 (0·06-0·12 BOD). At loading rates below about 0·17 g SS g MLSS-1 d-1 (0·07 g BOD g MLSS-1 d-1) the mixed liquors were acidic, with pH values down to 5·2, whereas at loading rates above about 0·80 (0·32 BOD) they were alkaline, with pH values up to 8·9. At intermediate loading rates the mixed liquor pH value was more variable though in general the higher the loading rate the higher also the pH value of the mixed liquor. Acidic conditions in the mixed liquors were attributed to the occurrence of nitrification, while in the absence of nitrification the mixed liquors remained alkaline. The concentration of BOD5 in the supernatant from the mixed liquors increased with increasing loading rate from about 35 mg 1-1 at a loading rate of 0·17 g SS g MLSS-1 d-1 (0·07 g BOD g MLSS-1 d-1) to about 250 mg 1-1 at a loading rate of 1·30 (0·52 BOD). The output of suspended solids from the treatment systems represented about 70 per cent of input suspended solids at loading rates of about 0·15 g SS g MLSS-1 d-1 (0·06 g BOD g MLSS-1 d-1) and increased to about 100 per cent at loading rates of 0·80 (0·32 BOD). Output of chemical oxygen demand was about 60 per cent of input at the lower loading rates and 80-90 per cent at the higher ones. Operation of treatment units at temperatures of 5 and 10°C instead of 15°C had little effect on the efficiency of degradation at loading rates in the range 0·085-0·20 g SS g MLSS-1 d-1 (0·034-0·08 g BOD g MLSS-1 d-1), but nitrification was prevented at 5°C. At loading rates of 0·77 (0·31 BOD) and 1·46 (0·58 BOD) operation at 25°C appeared to increase the amount of degradation as compared with that achieved at 15°C. The practical implications of the results and possible future approaches to the aerobic treatment of farm wastes are discussed.