Specific surveys of the air for Aspergillus fumigatus were carried out in rural and urban situations over a 2-year period. Overall, low concentrations of spores were recorded with a higher incidence during the "winter" months. Counts in the open air and in a hospital ward showed similar fluctuations, the indoor counts being consistently lower. Plant debris in the form of compost heaps and stacks of hay and straw baled with a high moisture content in which self-heating occurs, produces large numbers of spores which may be liberated into the air causing high but localized counts if disturbed. The widespread distribution of decaying leaves following leaf fall represents a potential source of smaller concentrations of spores but over a much larger area. This availability of decaying plant debris with high water content fulfils the growth requirements of Aspergillus fumigatus and is the probable explanation of its winter seasonality.