The problem of echo disruption has been frequently noted by authors who have studied parasites of harbor porpoise. Six harbor porpoises were examined macroscopically and microscopically for the occurrence of Stenurus minor. The present investigation confirmed earlier observations that this nematode may probably cause auditory malfunction, as there were changes to the auditory nerves of the cochlea and middle ear in consequence of chronic or acute inflammation with parasitic infection in five animals. Moreover, it can be stated that damage caused by S. minor produced severe stress and pain, which, very likely, affected the animal behavior.