The Photorhabdus luminescens W14 toxin encoding gene makes caterpillars floppy (mcf) was discovered due to its ability to kill caterpillars when expressed in Escherichia coli. Here we describe a homologue of mcf (renamed as mcf1), termed mcf2, discovered in the same genome. The mc/2 gene predicts another large toxin whose central domain, like Mcf1, also shows limited homology to Clostridium cytotoxin B. However, the N-terminus of Mcf2 shows significant similarity to the type-III secreted effector HrmA from the plant pathogen Pseudomonas syringae and no similarity to the N-terminus of Mcf1. HrmA is a plant avirulence gene whose transient expression in tobacco cells results in cell death. Here we show that E coli expressing Mcf2 can, like E coli expressing Mcf1, kill insects. Further, expression of the c-Myc tagged N-terminus of Mcf2, the region showing similarity to HrmA, results in nuclear localisation of the fusion protein and subsequent destruction of transfected mammalian cells. The Mcf1 and Mcf2 toxins therefore belong to a family of high molecular mass toxins, differing at their N-termini, which encode different effector domains.