Lysinibacillus sphaericus is used for the biological control of mosquitoes. The main toxicity mechanism of pathogenic strains is a binary toxin produced during sporulation. S-layer is a proteinaceous structure on the surface of bacteria; its functions have been involved in the interaction between bacterial cells and the environment, for example, as protective coats, surface recognition, and biological control. In L. sphaericus, S-layer protein (SlpC) is expressed in vegetative cells, and is also found in spore-crystal preparations; it has larvicidal activity in Culex spp. In this study, partial and completed sporulated culture toxicities were compared; also, S-layer protein and spore-crystal proteins were tested against Culex quinquefasciatus larvae for possible interactions. Larvicidal activity obtained with a combination of SlpC and spore-crystal proteins from strain III(3)7 showed no significant interaction, whereas, combinations of both preparations from strain 2362 showed synergistic effect. The highest synergistic activity observed was between spore protein complex from strain 2362 and SlpC from III(3)7. S-layer protein could be considered a good alternative in formulation improvement, for biological control of mosquitoes.