Extrinsic allergic alveolitis, with acute attacks of the disease, developed in a sawmill worker. The diagnosis was based on the history of exposure to moldy wood, on the symptoms and clinical signs, and on physiological and radiographic findings. A microbiological analysis of the ambient air of the patient`s work environment showed that the air contained a considerable number of spores, both of fungi and actinomycetes. The most common fungal genus was Aspergillus. Thermoactinomyces vulgaris predominated the actinomycetal flora. The patient`s serum contained precipitins to Thermoactinomyces vulgaris and Aspergillus fumigatus. Based on the microbiological and serological data, it was concluded that the causative agent(s) of the disease was either T vulgaris or species of the genus Aspergillus.