Zygocin, a protein toxin secreted by virus-infected strains of the osmotolerant spoilage yeast Zygosaccharomyces bailii kills a broad spectrum of yeast and fungi of medical and agricultural relevance. Its mode of action requires initial binding to a toxin receptor in the fungal cell wall, but finally unfolds at the level of the cytoplasmic membrane. In the infected host, toxin expression is mediated by a 2.1 kb viral double-stranded (ds)RNA genome encoding a prepro-zygocin precursor, which is further processed to the biologically active toxin (zygocin) by endopeptidase Kex2p cleavage within the yeast secretory pathway. cDNA sequence of the toxin-coding dsRNA genome shows no significant homology to other protein toxins, or to any protein known so far. In vivo toxicity involves membrane permeabilization and critically depends on an energized susceptible host cell. Possible mechanisms of toxin resistance involve chromosomal host genes whose gene products affect plasma membrane lipid composition. Functional and structural properties of zygocin resemble those of antimicrobial peptides from higher eukaryotes and indicate a potential of this unique virus toxin as novel antimycotic drug.