We have found a novel mycovirus, MoCV1 in the rice blast fungus, Magnaporthe oryzae. MoCV1 has five dRNA segments as genome, and belong to Chrysoviridae tentatively. Using micro-spin column method or one-step reverse-transcription PCR (RT-PCR) assay, we detected a MoCV1-related virus from M. oryzae in Japan, whose sequence shares considerable identity with that of the MoCV1 Vietnamese isolate. To establish a system for comprehensive survey of MoCV1 infection in the field, we developed a reverse-transcription loop-mediated isothermal amplification (RT-LAMP) assay for direct detection of the virus. In this review, we introduce our current knowledges of MoCV1 properties for biochemical and molecular genetic aspects and also describe its negative effects to host fungus, which imply potentiality to utilize MoCV1 as bio-controller. Heterologous gene-expression system in yeast is employed to investigate biological activities or functions of mycoviral proteins in fungal host cells. MoCV1-A infection caused hypovirulence to the host fungus, unexpectedly, also resulted in the change of pathogenic races in several differential rice lines, namely S (compatible) to R (incompatible) reaction or R to S. The cause of epigenetic alteration is also discussed.