Study of the air spora of a cowshed by means of a Hirst Automatic Volumetric Spore Trap showed an atmospheric concentration of fungal spores ranging from 95,000 to 16,000,000 spores/m3. There was a direct relationship between the hours during which hay was being fed and the highest concentrations of spores. Aspergillus-Penicillium and Mucor types of spore were predominant, and hyphal fragments including conidiophores were the third most numerous component. The findings are discussed with reference to human and animal fungal disease.