Cetaceans and, in this particular case, harbour porpoises Phocoena phocoena, rely on hearing for vital activities, and any perturbations to sound perception can have significant impacts on their survival. Harbour porpoise cranial sinuses, and specifically peri bullar sinuses, are parasitized by the nematode Stenu rus minor (Metastrongyloidea: Pseudaliidae), Kühn, 1829 with high prevalence (Deliamure 1955, Arnold & Gaskin 1975, Brosens et al. 1996, Faulkner et al. 1998, Siebert et al. 2001, 2006, Kijewska et al. 2003, Lehnert et al. 2005, 2014. At present, the effect of S. minor on the hearing ability of harbour porpoises is still controversial. While the prevalence of S. minor in pterygoid and peribullar sinuses and tympanic cavity was high in harbour porpoises (66.67% in our cases), its prevalence in the cochlea was rare. Only 1 case out of 129 analysed by either histology, electron microscopy or immunofluorescence showed the presence of a nematode parasite morphologically consistent with S. minor at the most basal portion of the right cochlea. This individual also had severe haemorrhage along the right cochlear spiral, which was likely caused by ectopic S. minor migration. Although this animal might have had impaired hearing in the right ear, it was otherwise in good body condition with evidence of recent feeding. These findings highlight the need to study the effect of parasites on hearing, and other pathological changes that might impair appropriate processing of acoustic information.