The effects of temperature, dissolved oxygen, and pH on the growth, sporulation, and mosquito larval toxin formation by B. sphaericus 1593, were investigated in shaken flasks and a fermenter. The bacteria grew well at temperatures of 25 to 40°C, however, toxin-formation and sporulation were poor at temperatures above 30°C. Cell lysis became evident after about 30 hours into the fermentation, particularly in case of high temperatures. Mature spores, as well as most of the toxin, were formed by 24 to 30 hours of growth. Controlling the pH at 7.0, gives about ten-fold more toxicity as compared to experiments with no pH control. Simultaneous assay of ammonia concentration indicated a close parallel between the ammonia and pH profiles. Assays of carbohydrate, protein, and phosphorous indicated that none of these nutrients were limiting at any time, thus none of them could have been the limiting nutrient that triggers sporulation. It was found that dissolved oxygen concentrations dropped nearly to zero in case of fermentation with sparged air, but were significant in the case of fermentation with sparged oxygen even at their minimum. The toxicity was approximately equal in both the cases, however sporulation was poor in the case of experiments sparged with pure oxygen. Measurements of oxygen uptake rates confirmed that the cells respire more when the cells are in the vegetative phase of growth. / M.S.