Bacillus cereus isolated from the larvae of Myrmeleon bore was found to secrete proteins that paralyze and kill German cockroaches, Blattela germanica, when injected. One of these active proteins was purified from the culture broth of B. cereus using anion-exchange and gel-filtration chromatography. The purified toxin, with a molecular mass of 34 kDa, was identified as sphingomyelinase C (EC 3.1.4.12) on the basis of its N-terminal and internal amino-acid sequences. A recombinant sphingomyelinase C expressed in Escherichia coli was as potent as the native protein in killing the cockroaches. Site-directed mutagenesis (His151Ala) that inactivated the sphingomyelinase activity also abolished the insecticidal activity, suggesting that the rapid insect toxicity of sphingomyelinase C results from its phospholipid-degrading activity.