Aspergillus fumigatus (A. fumigatus) is the most common fungal pathogen found in cystic fibrosis (CF) airways, although its prevalence has until recently been underestimated. Colonisation and infection with A. fumigatus is associated with a decline in lung function in CF patients, but itâs precise role in disease progression remains unclear. Airway mucus is the first point of contact for A. fumigatus spores following inhalation, providing an important site for hostpathogen interactions. The interplay between A. fumigatus and the mucus barrier has only recently begun to be studied, and its effects on the respiratory mucins MUC5B and MUC5AC are yet to be elucidated. Through the use of agarose gel electrophoresis and rate zonal centrifugation, an in depth analysis of the effects of A. fumigatus on purified mucins was performed. Treatment of MUC5B and MUC5AC with A. fumigatus culture filtrates led to a reduction in mucin size over time. Recombinantly expressed mucin domains were used to identify potential cleavage sites within MUC5B, identifying the D1 and D3 domains of the N-terminus and CK domain of the C-terminus as regions targeted by secreted proteases. Lectin assays were used to determine the effects of A. fumigatus on mucin glycosylation, with data suggesting cleavage of mucin glycans by glycosidase enzymes. Using protease inhibitors and a panel of A. fumigatus secreted protein knockouts, Alp1, signal peptidase I, DppV and Mep were identified as putative mucin-degrading proteases. Knockout strains for the cell-surface receptor PalH and the transcription factor PacC identified these proteins as regulators of mucin degrading proteases, with culture filtrates from mutant strains showing a reduced ability to degrade MUC5B and MUC5AC. Through the use of purified CF mucins, the potential effects of A. fumigatus on CF airways was investigated. Mucins from patients testing positive for A. fumigatus appeared smaller than those from A. fumigatus negative patients, with mucin degradation correlating with a reduction in mucin viscosity. Using a repeated exposure in mice it was shown that A. fumigatus may cause an increase in intracellular MUC5B levels and extracellular MUC5AC levels. Finally, our data suggest that mucins alone are not sufficient to act as a sole carbon source to support A. fumigatus growth. Overall, these data provide a useful insight into the potential role of A. fumigatus in CF, highlighting mucin degradation as a potential mechanism of disease pathogenesis.