Mass mortalities due to disease are important determinants of population and community structure in marine ecosystems, but the speed at which an epizootic may sweep through a population, combined with rapid selection for disease-resistant stocks, can mask the ecological impact of disease in all but the most closely monitored populations. We document an emergent epizootic event in the deep sea that is occurring in mussels (Bathymodiolus brevior) at the Mussel Hill hydrothermal vent in Fiji Basin and we identify the causal agent as a black yeast (order Chaetothyriales) that elicits a pronounced host immune response and is associated with tissue deterioration. The yeast was not observed in other invertebrate taxa (the gastropods Ifremeria nautilei, Alviniconcha aff. hessleri; the limpets Lepetodrilus schrolli, Symmetromphalus aff. hageni; the polychaetes Branchipolynoe pettiboneae, Amphisamytha cf. galapagensis) associated with the mussel bed, nor in mussels (Bathymodiolus brevior) collected from adjacent Lau Basin mussel beds. Massive mussel mortality resulting from the fungal infection is anticipated at the Mussel Hill site in Fiji Basin; we expect that epizootic outbreaks in dense invertebrate communities have the potential to be major determinants of community structure in deep-sea chemosynthetic ecosystems. The possibility that submersible assets may serve as vectors for transport of the fungus warrants further attention.