The main toxicity mechanism of Lysinibacillus sphaericus, which is used in the control of mosquitoes, is its binary toxin produced during sporulation; additionally the Mtx1, Mtx2 and Mtx 3 toxins are expressed in vegetative cells. Mosquito larvicidal potency of the S-layer protein that is expressed in vegetative cells has been determined. The protein is similar to other S-layer proteins of mosquitocidal L. sphaericus strains. The LC50 values of the S-layer protein of the L. sphaericus OT4b25, OT4b26, and III(3) 7 strains against third-instar larvae of Culex quinquefasciatus were 8.7, 24 and 0.68 mu g/ml, respectively. To our knowledge this is the first study showing the mosquito larvicidal potency of the S-layer protein from Lysinibacillus sphaericus.