Sludge characteristics of a full-scale vacuum rotation membrane (VRM) bioreactor having plate-type membranes with 0.038 m nominal pore size and 540 m(2) surface area were investigated. The VRM plant is composed of an aeration tank and a filtration chamber. The sludge floc size distribution, as determined microscopically, was mainly between 0 and 100 m in the filter chamber with very little difference in size distribution between summer (20-25 degrees C) and winter (10-15 degrees C) seasons. Small floc size encountered in the filtration chamber was evident due to excessive shear administered by the rotation action and vigorous agitation by the scouring air. The mixed liquor suspended solids (MLSS) was variable between 5.23 and 14.10 g/L in the VRM tank and sludge behaved plastic. Viscosity ranged between 2.39 and 30 cP in the VRM tank in relation with MLSS concentration and temperature. Above 20 cP inter-plate spaces rapidly clogged leading to major clean-up.