A correlation between spore productivity and k(L)a, was found for Bacillus thuringiensis var. kurstaki. Since insecticidal crystal proteins synthesis depends on the highly oxygen-dependent sporulation process, the fermentation process was scale-up on the basis of the product k(L)a p, where p is the fermenter total pressure. The process was scale-up from a 14-l fermenter to a 1,100-l fermenter. Both reactors did not have the same geometrical design. Even though the specific growth rate decreased with the change of scale, the scale-up had a positive effect, since the total fermentation time was decreased white the biomass yield and the sporulation efficiency were unchanged. Therefore k(L)a p is a practical tool for scaling up B. thuringiensis processes where product synthesis depends on sporulation.