Alginate microspheres prepared by internal gelation were chosen as carrier for a model microbial larvicide, Bacillus thuringiensis (Bt), owing non-toxicity of the polymers and mild conditions of the method. The coated colloids were characterized using optical microscope and scanning electron microscopy (SEM). The photostabilization of unencapsulated and encapsulated Bt powders when exposed to ultraviolet light was evaluated. The effect of the Bt-loaded alginate microspheres on the biology and mortality of Martianus dermestoides was investigated in the laboratory. The results showed that Bt-loaded alginate microspheres which were obtained by association had a mean length of 135.8 ± 15.8 µm. The photolysis experiments demonstrated that the UV light-induced decomposition of Bt was markedly decreased by encapsulation into microspheres. Therefore, it can be considered an effective system to enhance the photostability of Bt. Increased persistence against high temperature was also observed. Toxicity of the novel Bt-loaded alginate microsphere insecticide was higher in the adult stage compared to the unencapsulated Bt as indicated by the lower LC50 value.