Sludge filterability in membrane bioreactors (MBRs) fluctuates and affects membrane fouling. Therefore, understanding the reasons for fluctuation of sludge filterability is important for efficient operation of MBRs. In this study, a pilot-scale MBR treating municipal wastewater was operated for about 600 days and variations of sludge filterability were continuously monitored by batch-filtration experiments using the same membranes equipped in the MBR. To investigate reasons for deterioration of sludge filterability, constituents in sludge supernatant were intensively monitored, and the correlations with sludge filterability were determined. The concentration of lipopolysaccharides (LPS) in sludge supernatant exhibited significantly higher correlation with sludge filterability than did conventional indexes (i.e. polysaccharides and proteins). Size fractions affecting MBR sludge filterability were also investigated, and it was suggested that colloidal LPS deteriorated MBR sludge filterability. Based on a long-term operation of the MBR, increase in colloidal LPS under low temperature of mixed liquor suspension was a key factor of deterioration of sludge filterability. Impact of LPS increased under low temperature should be investigated by operating bench-scale MBRs fed with synthetic wastewater under a controlled condition.