Two laboratory-scale anaerobic hybrid reactors, R1 and R2, treated low- (1 kg COD m-3) and high-strength (10 kg COD m-3) whey-based wastewaters, respectively, in a 500-day trial. The chemical oxygen demand (COD) removal efficiencies of R1 averaged 70-80%, at organic loading rates of 0.5-1.3 kg COD m-3 day-1, between 20 and 12 degrees C. The COD removal efficiencies of R2 exceeded 90%, at organic loading rates up to 13.3 kg COD m-3 day-1, between 20 and 14 degrees C. Lowering the operating temperature of R2 to 12 degrees C resulted in a decrease in COD removal efficiency, to between 50% and 60%, and a disintegration of granular sludge. The decline in performance, and granule disintegration, was reversed by decreasing the organic loading rate of R2 to 6.6 kg m-3 day-1. Specific methanogenic activity profiles revealed mesophilic (37 degrees C) temperature optima for biomass in both reactors, even after 500 days of psychrophilic operation, although the development of psychrotolerance in the biomass was noted.