Anaerobic membrane bioreactors (AnMBRs) have been shown to be successful units for the treatment of low strength wastewaters, however, the issue of membrane fouling is still a major problem in terms of economic viability. Biogas sparging has been shown to reduce fouling substantially, and hence this study monitored the effect of biogas sparging rate on an AnMBR. The critical flux under a sparging rate of 6 l per minute (LPM) was found to be 11.8 l m(-2) h(-1) (LMH), however, membrane hysteresis was found to have an effect on the critical flux, and where the AnMBR had previously been operated with a 2 LPM sparging rate, the critical flux fell to 7.2 LMH. The existence of a "critical sparging rate" was also investigated under the condition that 'there exists a sparging rate beyond which any further decrease in sparging rate will cause a dramatic rise in TMP'. For an AnMBR operating at a flux of 7.2 LMH the critical sparging rate was found to be 4 LPM.