Delta-endotoxin production by a strain of Bacillus thuringiensis subsp hurstahi exhibiting larvicidal toxicity towards lepidoptera was investigated in 2- and 20-dm(3) fermenters, using gruel- and fish meal-based media. The results show clearly that in such complex media, aeration plays an important role in bioinsecticide production. Optimal aeration led to improvement of delta-endotoxin concentrations with decreases of final spore count and proteolytic activity. Moreover, in order to use high gruel concentrations, a fermenter configuration with an efficient aeration system should be used. In a 20-dm(3) Biolafite fermenter, 59 g dm(-3) or 75 g dm(-3) gruel was used to produce bioinsecticides with a significant reduction of carbon catabolite repression of delta-endotoxin synthesis. This result is very interesting in order to produce high final delta-endotoxin concentrations in the culture broth. It was also concluded, by considering the key role of oxidative pathways in delta-endotoxin synthesis, that oxygen supply must be adequate for bioinsecticide production at high substrate concentrations. Moreover, the role of sodium chloride in improving delta-endotoxin production is dependent not only on protease synthesis and its effect on crystal stability, but also on the aeration level of the production medium.