Delta-endotoxin production capacity of several Bacillus thuringiensis (Bt) strains exhibiting various larvicidal activities towards either lepidoptera or diptera was investigated in gruel and fish meal media. Diptera-specific strains produced less delta-endotoxins (1246-1998 mg l(-1)) than lepidoptera-specific ones (3060-3301 mg l(-1)). Carbon catabolite repression regulated delta-endotoxins synthesis for all strains, but no relationship existed between the specificity of delta-endotoxins and their sensitivity to such repressive regulation. The production of proteolytic activities by the isolate BNS3 and the commercial strain HD1, was reduced in 0.5 g l(-1) sodium chloride and 0.1% Tween-80 but not by other strains. Moreover, in the case of the diptera-specific strain BUPM98, there was a need for additional nitrogen sources and to some extent, several amino acids from yeast extract, leading to an increase of 22% in delta-endotoxin production. Sporulation of BUPM98 was never reached in the rich Luria Broth (LB) medium, compared with that of the lepidoptera-specific strain BNS3. In shake flask cultures at 10 g l(-1), sodium acetate notably increased (38-79%) delta-endotoxins production by the diptera-specific strains, with a strong decrease in protcolytic activities. Similar delta-endotoxin production and high proteolytic activities were obtained with or without 10 g l(-1) sodium acetate with an excess of aeration in a 2-1 fermenter. In a less aerated medium, a strong effect of acetate was observed, decreasing production of proteolytic activities and exhibiting high catabolite repression on delta-endotoxin synthesis. The effect of sodium acetate was not directly related to sporulation and protease synthesis but to acetate metabolism. Culture conditions were defined for the screening of Bt strains producing high counts of delta-endotoxins specific to either lepidoptera or diptera.