Bacillus thuringiensis subsp kurstaki was grown in batch and fed batch cultures using wastewater sludge as a raw material. A simple fed batch strategy based on dissolved oxygen measurement during the fermentation cycle was developed in this work. It was established that while shifting the process strategy from batch to fed batch, the maximum spore concentration was increased from 5.62 x 10(8) to 8.6 x 10(8) colony forming units per cm(3) and resulted in an increase of entomocidal activity from 13 x 10(9) to 18 x 10(9) spruce budworm potency units per dm(3). Higher entomotoxicity was recorded at low spore concentration using wastewater sludge as a raw material whereas low entomotoxicity was reported at high spore concentration in synthetic medium. (c) 2005 Society of Chemical Industry