AbstractAspergillus fumigatus has long been a focus of research, as it is the cause of the majority of Aspergillus infections. A. fumigatus is widely distributed in the environment and mainly distributed in air as conidia and is the main source of lung infection. A. fumigatus airborne counts were determined monthly during two years from the outside air environment at the University of Manchester campus and compared to total fungal airborne counts. Total fungal airborne counts were strongly seasonally associated with peak counts occurring during the summer months reaching 1,100-1400 CFU m-3and were correlated positively with mean temperature (R2=0.697). In contrast, Aspergillus fumigatus counts were not seasonally associated and gave persistent low levels of between 3-20 CFU m-3and were not correlated with mean temperature. A random selection of Manchester environmental isolates collected over one year along with clinical patient isolates and environmental isolates from the air from Dublin were analysed for genetic diversity using two combined RAPD primers. RAPD analysis revealed that the Manchester environmental isolates represented a genetically diverse population while the clinical isolates were less diverse and formed three major clusters. The Dublin isolates were the least diverse, probably due to their isolation at a single time point. When the pathogenicity of clinical and Dublin isolates were compared with a random selection of Manchester isolates in a wax moth model, as a group, clinical isolates were significantly more pathogenic than environmental isolates. Moreover, when relative pathogenicity of individual isolates was compared, clinical isolates were the most pathogenic, Dublin isolates the least pathogenic and Manchester isolates showed a range of pathogenicities suggesting that selection for the most pathogenic isolates from the environment occurs during patient infection. When the expression of secreted phospholipases in vitro during wax moth larvae of a range of isolates displaying varying degrees of pathogenicity was compared, two phospholipase C genes, AfplcA and AfplcC were strongly correlated with pathogenicity. AfplcC was by far the most highly expressed, however a DeltaAfplcC knockout strain did not show attenuated virulence compared to the wild type in wax moth larvae.