The species historically referred to as Buellia stillingiana, and more recently proposed to be a synonym of B. erubescens, is very common and widespread in eastern North America. While studying material to determine the application of these names, we repeatedly encountered a lichenicolous coelomycete parasitizing thalli of this lichen. The lichenicolous fungus appears to belong to the genus Minutophoma, differing from M. chrysophthalmae in its larger pycnidia, larger conidia, and occurrence on Buellia rather than Chrysothrix. It is here described as M. buelliarum. After studying more than one thousand specimens of Buellia stillingiana, M. buelliarum was found, albeit rarely, throughout the range of this lichen from the island of Newfoundland in eastern Canada south throughout the Coastal Plain and west to the edge of the Great Plains. It was also found to occur on other morphologically similar sympatric Buellia species producing norstictic acid: B. curtisii, B. imshaugiana, and an unidentified Buellia species similar to B. stillingiana. Keywords: Biodiversity, BiotaNB, biotic interactions, Buellia stillingiana, B. curtisii, B. imshaugiana, B. erubescens, host-parasite range dynamics, parasite, taxonomy.