Bacillus thuringiensis (Bt) production using wastewater sludge as a raw material was studied in bench scale fermentor at different sludge solids concentration. The highest cell and spore concentrations and entomotoxicity were observed at a suspended solids concentration of 20-25 g/l. Maximal protease activity also increased with solids concentration, however, it decreased after attaining a peak at about 36 h. Maximal protease activity (2.3-2.8 IU/ml) obtained at 25 and 20 g/l sludge suspended solids concentration was 1.75 and 1.5 times that of the maximal activity obtained at 15 g/l. Irrespective of the sludge suspended solids concentration, protease activity (until its peak) showed an exponential relationship with total cell count and a linear relationship with entomoxicity potency. An exponential correlation was also established between spore concentration and entomotoxicity. The correlations tested using a regression model were highly significant. (c) 2005 Elsevier Ltd. All rights reserved.