Killer protein 4 (KP4) is a well studied viral toxin secreted by the maize smut fungus Ustilago maydis that kills sensitive Ustilago strains as well as inhibits Fusarium and plant root growth by inhibiting calcium uptake. Numerous small, cysteine-rich proteins have been shown to play a critical role in fungal-plant-bacterial associations. The discovery of six KP4-like genes in F. verticillioides precipitated efforts to understand their function and evolutionary origin. Analysis of publicly available genomic sequence identified 31 additional KP4-like genes from a range of Ascomycota, a Basidiomycota, and the moss Physcomitrella patens. Sequence comparison and phylogenetic analysis indicate that the viral KP4 and the moss and fungal KP4-like genes evolved from a common ancestor providing evidence for lateral gene transfer between kingdoms. Six genes of the 37 total genes are predicted to encode a protein with two, non-identical KP4-like domains in tandem separated by 29-56 amino acids. The results suggest that two independent events led to the dual-domain KP4 genes present in different lineages of the Ascomycota. Understanding the nature and function of KP4-like proteins in mycotoxin-producing species like Fusarium may help to limit plant diseases and increase food safety and food production.