Certain strains of Bacillus sphaericus produce a highly toxic mosquito-larvicidal protein during sporulation which is active against vectors of dengue, encephalitis and malaria. This toxin is initially expressed as 51 and 42 kDa proteins and is converted to 43 and 39 kDa proteins, respectively, which form the active heterodimer complex. For a better understanding of the toxicity mechanism at the molecular level, the 51 kDa protein of the binary toxin of B. sphaericus strain 2297 was expressed as a glutathione-S-transferase fusion protein and purified by affinity chromatography. Protein crystals were grown from an amorphous precipitate in five months using the hanging-drop vapor-diffusion method. The protein crystals were dissolved and were found to be composed of a proteolytically modified 45.2 kDa derivative similar to the active form of this protein. The crystals form in space group P43212 (or P41212) and diffract to 2.6 A, with unit-cell dimensions a = b = 133.48, c = 69. 76 A.