Under northern climatic conditions, a temporary decrease in the temperature of anaerobic reactors treating swine manure is likely to happen at the farm. The objective of this study was to evaluate the impact of temperature fluctuations, between 10 and 20 degreesC, on the stability and performance of psychrophilic anaerobic sequencing batch reactors (ASBRs) treating swine. manure.Methane yield decreased from 0.266 +/- 0.014 l/g of total chemical oxygen demand (TCOD) fed to the ASBRs at 20 degreesC to 0.218 +/- 0.022 and 0.080 +/- 0.002 l/g TCODfed at 15 and 10 degreesC, respectively. Soluble chemical oxygen, demand (SCOD) reduction decreased from 94.2 +/- 1.1% at 20 degreesC to 78.8 +/- 3.0% at 15 degreesC and 60.4 +/- 6.4% at 10 degreesC. Total COD removal also tended to decrease as temperature was lowered, but difference between operating temperatures was not as pronounced. A lower methanogenic activity in the ASBRs operated at 10 degreesC probably favoured quiescent conditions during the settling period, thereby increasing physical removal of the TCOD through sedimentation of the solids with the biomass. When the operating temperature was increased back to 15 and 20 degreesC, methane yield and SCOD reduction improved, but reactor performance remained significantly (P < 0.05) lower than that achieved before the cycles at 10 degreesC. Results from this experiment nevertheless suggested that fluctuation in the operating temperature of psychrophilic ASBRs should only have temporary effects on the performance and stability of the process. Crown Copyright (C) 2003 Published by Elsevier Science Ltd. All rights reserved.