The effect of a surface active agent, Tween 80 (0.2%, v/v) on production of Bacillus thuringiensis (Bt) based biopesticides using secondary wastewater sludge (non-hydrolysed (NH) and hydrolysed (TH)) as a raw material was studied in bioreactors. Hydrolysed sludge exhibited higher entomotoxicity (Tx) (49% increase) and cell/spore concentration vis-a-vis non-hydrolysed sludge. Amending the non-hydrolysed sludge with Tween 80 resulted in increase in cell and spore count by 1.67 and 4 times respectively, maximum specific growth rate (mu(max)) increased from 0.19 to 0.24 h(-1) and Tx increased by 26.6%. However, addition of Tween 80 to hydrolysed sludge increased cell and spore count only by 2- and 2.4-folds, respectively and mu(max) increased from 0.28 to 0.3 h(-1) with no change in Tx. Volumetric mass transfer coefficient varied significantly during fermentation and oxygen uptake rate (OUR) also increased 5 and 3.5 times for Tween 80 fortified non-hydrolysed and hydrolysed sludge, respectively. There were variations in viscosity during fermentation due to concerted effects of increase in viable cell and spores, cell lysis, agitation, aeration and anti-foam addition. The particle size decreased markedly in all cases at the end of fermentation. (c) 2005 Elsevier Ltd. All rights reserved.