During this work the fermentation of Bacillus thuringiensis var israelensis under industrial conditions was studied with respect to the development of a process for the production of a mosquitocidal insecticide elaborated by this organism. This was done by the development of a two-stage inoculum protocol which produced a high biomass-containing inoculum of vegetative cells which were found to be preferable to free spores for use as an inoculum source. In order to optimize the production stage fermentation, a range of 1iterature-reported media were studied with respect to cost, sporulation efficiency, biomass production, and fermentation time. The components of these media were then varied and the variant media studied with respect to the same parameters. Finally, all media were examined in relation to their bioactivity (insect toxici ty) production capability, which is the most important parameter of the fermentation of B. thuringiensis. In order to be able to estimate the bioactivity of the culture broths, a mosquito rearing and bioassay facility , as well as *a guinea pig colony, were established. This series of flask-scale investigations highlighted the soyabean/molasses (SM) medium as the most cost-effective medium studied. Following the flask-based experiments, the second inoculum and production stages were scaled up to laboratory-scale fermenters, and then the production stage was run on a pilot-scale. In addition to these fermentation studies, the purification of the parasporal crystals by an aqueous biphasic separation technique was studied with a view to assessing this method for the production of an organism-free insecticide formulation. An alternative, shorter and less labour-intensive, bioassay was also investigated. Finally, a study of proteases produced by B. thuringiensis var israelensis was undertaken in order to determine the number,, types, and time of production of these enzymes during the growth of the organism in industrial media.