Yeast cells occasionally carry “killer” viruses, which are capable of producing and secreting to the environment toxins that kill cells that do not carry such viruses. It is believed that killer yeasts are common based on the fact that they have been found among yeasts isolated from different sources over several decades. Presumably, toxic killing provides benefits to the yeast host. However, the ecological and evolutionary significance of toxin production remains poorly understood. For example, it is unknown where yeast killers occur, how evolvable killing ability is, and how it is shaped by interactions with toxin-sensitive cells. Also unknown is how stable yeast-virus symbioses are, and how coevolution between host and virus may affect this stability and the killing phenotype itself. My studies show the usefulness of the killer yeast to address questions related to competition and coevolution, which may proof valuable also given potential applications of killer yeasts in the fermentation industry.