The effect of shear strength measured in agitation rate ranging from 50 to 800 rpm on physical properties and metabolism of microbial cells was investigated using activated sludge floes. The response of microbial cells to varying external shear strength was monitored in terms of sludge particle size, settling properties, cell surface hydrophobicity, and metabolic behavior measured as carbon dioxide (CO2) production, oxygen (O-2) utilization rate, and sludge production rate. Exopolysaccharide production of the microbial cells increased with shear strength, while the sludge production rate decreased with the shear strength. In addition to stimulate CO2 production, shear strength could also induce the change of cell surface hydrophobicity. A higher cell surface hydrophobicity, larger floe size, and lower sludge volume index (SVI) were observed at mild shear strength when sludge floes were agitated at a rate of 400-600 rpm. The results suggested that the metabolic behavior of microbial cells may change in respond to applied shear strength, which results in a subsequent change in physical properties of the sludge floes. It can be concluded that mild shear strength is likely to exert a favourable influence on the cell attachment, leading to the growth of settable biological floes. (c) 2005 Elsevier Ltd. All rights reserved.