Membrane bioreactors (MBRs) have proven to be a valuable alternative for conventional activated sludge plants. Membrane fouling remains, however, a significant drawback. Stable operation of a membrane bioreactor requires a good sludge condition. In this paper it is evidenced that bioflocculation is a crucial factor within that context. By changing the ratio of monovalent over polyvalent cations ([M/P]) in the influent, a deflocculation-reflocculation event was induced during which the impact of bioflocculation on fouling in membrane bioreactors could be studied. In a first phase, a high [M/P] influent was fed to the MBR which resulted in severe sludge deflocculation and worsened filtration characteristics. A low [M/P] influent was subsequently fed to the MBR. Within 3 weeks, the activated sludge reflocculated and filtration characteristics improved significantly. Monitoring of bioflocculation in MBRs is thus of extreme importance to start possible remediation as quickly as possible. Fragment surface proved to be a valuable parameter in that respect. In contrast, no clear relation between EPS and filtration characteristics could be found. This work substantiates the hypothesis that a well functioning dynamic secondary membrane, built up by robust activated sludge flocs, can prevent severe (irreversible) fouling in MBRs. (C) 2009 Elsevier B.V. All rights reserved.