Hearing is one of the major senses in whales and dolphins (cetaceans). This is the first report of severe mycotic otitis media in a cetacean, a juvenile female harbour porpoise (Phocoena phocoena) from British waters that stranded alive. Gross examinations were followed by histological and microbiological investigations of the auditory apparatus. Both tympanic cavities and periotic sinuses displayed copious greenish-yellow purulent and caseous material. Severe fungal infestation by Aspergillus terreus was documented in the otic region but not in any other site of the body. Adjacent to the promontorium, massive accumulation of fibrinous secretion and infiltration of clusters of inflammatory cells were present. Newly formed cysts and vessels replaced the round window membrane location, reminiscent of granulation tissue. Inflammatory cells and a severe fibrin net were noted within the perilymphatic spaces of scala tympani and scala vestibuli, indicative of an acute fibrinous otitis. Inflammatory reactions have probably been caused by this fungal organism. The basilar membrane was solely covered by a simple cuboidal epithelium. Complete absence of sensory cells of the Organ of Corti characterised a further severe phenomenon, which possibly led to the animal’s poor nutritional status and stranding. Potential portals of entry are being discussed.