Emergent viral diseases are often a result of host-switching, which has conventionally been thought a rare phenomenon. Host-switching in fungal RNA viruses(mycoviruses) is thought to be especially rare since mycovirus transmission is presumably restricted to intracellular strategies. Extracellular transmission is the primary transmission strategy of nearly all other viruses however, and we hypothesize that the absence of extracellular transmission in mycoviruses is unlikely. Further, recent evidence demonstrates that host-switching is more frequent in RNA viruses than previously recognized. Are viruses in fungi really an exception to this? If so, how do they defend themselves from extracellular invasion? Whether, and to what extent, host-switching of mycoviruses occurs in nature has not yet been tested. If mycoviral transmission is as restricted as currently thought, theory predicts coevolution of mycovirus and host. Thus, we take a phylogenetic approach to test our hypothesis that host-switching of mycoviruses occurs. We are collecting soils in Michigan from environments that are similar in composition but spatially distant. Common fungal species will be selectively isolated from each soil, screened for mycoviruses, and the phylogenetic relatedness of mycoviruses infecting those fungi will be compared. If mycoviruses from the same soils, regardless of fungal host, are more genetically similar than viruses in the same species from different soils, then host-switching is supported. The guiding questions of this research address our basic understanding of disease and have implications important to human, wildlife, agricultural, and ecosystem health.