Growth, sporulation, synthesis of delta-endotoxins, and toxicity against the larvae of Aedes aegypti and Culex pipiens were studied during fermentation of Bacillus thuringiensis H14 in a 20-L fermentor. Measurements of optical density and dielectric permittivity for biomass determination suggest a highly promising technique for on-line evaluation of sporulation. The synthesis of 65-, 25- and 130-kDa proteins started at 16, 18, and 23 h, respectively. These proteins were enriched in different ways until the end of culture (48 h). Toxicity in the course of sporulation was significantly different for the larvae of both mosquito species. Maximal activity against Ae. aegypti was obtained at the end of culture, whereas for Cx. pipiens, the sample at 38 h was the most active.