Photorhabdus bacteria produce a number of toxins to kill their insect hosts. The expression of one of these, Makes caterpillars floppy (Mcf), is sufficient to allow Escherichia coli to persist within and kill caterpillars. Mcf causes shedding of the insect midgut epithelium and destructive blebbing of haemocytes suggesting it may trigger apoptosis. To investigate this hypothesis, here we examine the effects of E. coli-expressed Mcf on the mammalian cell lines COS-7, NIH 3T3 and HeLa cells. Cells treated with Mcf show apoptotic nuclear morphology, active caspase-3, DNA laddering after 6 h, and the presence of cleaved PARP after 16 h. These effects are prevented by the apoptosis inhibitor zVAD-fmk. Transfection of cells with constructs expressing only the NH2-terminal 1280 amino acids of Mcf, as a fusion with Myc, also triggered cell destruction. The expressed fusion protein was concentrated into the Golgi apparatus before cell death. These results confirm that the novel insecticidal toxin Mcf induces apoptosis but the precise intracellular pathway remains obscure.