Infection of the nose and accessory nasal sinuses by Aspergillus fumigatus is a rare disease. In a careful survey of the literature I was able to find only three cases recorded in which the infection in the sinuses was definitely proved to be due to Aspergillus fumigatus and five cases of aspergillosis of the maxillary sinus of undetermined type. Aspergillus fumigatus was first described by Fresenius in 1775. This is the commonest Aspergillus and is often found in various cereals, straw, hay and similar substances. It is the species most frequently found in man, giving rise to an aspergillosis of various organs. The spores are very resistant. Aspergilli are generally saprophytes, but they may become parasites. The effects on the human organism are due in addition to mechanical irritation to toxins secreted by the fungi. These toxins have been found to act on the muscular and nervous system of dogs.