A 36-year-old woman was admitted to our hospital because of fever, dry cough, dyspnea on exertion and body weight loss in August 2000. Chest radiography and CT scanning showed diffuse ground glass opacity and small centrilobular nodules in the middle and lower lung fields of both lungs. Serum antibody against Trichosporon cutaneum was positive; and summer-type hypersensitivity pneumonitis was therefore initially diagnosed. Treatment with methylprednisolone and prednisolone decreased the symptoms, but the dyspnea reappeared when the patient was at home. Inspection of her house revealed the presence of fungi under the floor. After these were removed, her symptoms disappeared completely. The lymphocytic stimulation test of the peripheral blood was positive for the fungi, and it was therefore suggested that they were the cause of her hypersensitivity pneumonitis. The fungi were identified as Gyrodontium versicolor. This is the first report of hypersensitivity pneumonitis caused by Gyrodontium versicolor.