The present study recorded the variations in the body water content and its relation to haemolymph volume, density and pH of the greater wax moth (Galleria mellonella) across different developmental stages at different time intervals post injection with Bacillus thuringiensis kurstaki. Preliminary results concerning Bt toxicity at a sub lethal concentration indicated that pupae were more resistant while, the adults were highly susceptible. Results concerning physiological investigations indicated that larvae had more body water content than pupae and adults, while haemolymph volume and density increased directly with insect development. Haemolymph pH had the same values in all developmental stages. Bt-injection decreased the body water content and, in the same time, increased the haemolymph volume, density and pH in all developmental stages. Maximum values attained at 48 h post injection. Pupae developed from previously treated larvae, and adults developed from previously challenged larvae and pupae showed significant changes compared with the normal ones and no changes as compared with treated insects.These findings show that different age groups respond to the impact of pathogenic bacteria by the same degree and the physiological changes did not affect the growth and transformation of this insect.