In one locality near Iowa City Armillaria mellia was attacked by a parasitic fungus which caused extensive hypertrophy of the upper region of the pileus of its host without rendering the gills sterile. The parasite developed a hymenium which spread over a series of thin, sharp-edged and highly convoluted ridges or gills. The microscopic characters show the fungus to be closely related to that which has been designated Exobasidium mycetophilum (Peck) Burt
