Otitis externa is the most common disease of the canine external ear canal and is defined as an inflammation of the canal, with an underlying reason in essentially all cases. The causes are defined as processes or factors that directly initiate the inflammation of the external ear canal. Dogs have three recognized species of Demodex mites, being the Demodex canis the most common. Also there is a short-bodied species and a large-bodied Demodex limited to the hair follicle and sebaceous gland, named Demodex injai. Of all these, only the D. canis is described as a cause of ceruminous otitis externa in the dog. Demodex injai mites, originally reported in 1997, is described as cause of erythema, scaling, greasy seborrhea and diffuse alopecia but never before described as cause of otodemodicosis. Thus, the aim of this work is to report the first cause of otodemodicosis due to Demodex injai mite. A 12-year-old cocker dog was presented at the veterinary teaching hospital with signs of chronic otitis, head shaking and ear pruritus that have beginning about a year ago. The animal has been treated with antibacterial-, antifungal- and corticosteroid-containing topic solution with no improvement. At the physical examination there was a nodule adjacent to the lips that were diagnosed as a carcinoma. Besides that, there were no signs of systemic or dermatologic disease except for the unilateral ceruminous otitis externa. The hematological evaluation and the biochemical profile showed no significant alteration. At the direct microscopic evaluation of cerumen was identified a form at least 50% bigger of Demodex mite, with no regular Demodex concurrent. The dog was treated daily with topical solution of ivermectin and mineral oil (1:19). After four weeks of treatment there were completely resolution of the ceruminous otitis and no mites at the microscopic examination were detected. The mites of D. injai is easily distinguishable from D. canis because adult males were approximately 100% longer and adult females were approximately 50% longer than adult male and female D. canis mites, respectively. It is suspected that in the adult-onset patients, concurrent immunosuppressive factors may be present to allow the previously controlled mites to proliferate excessively or there is a mite-specific immunoincompetence. In this dog, besides the carcinoma, there were no signs of immunosuppressive concurrent disease. Thus, the role of these potentially immunosuppressive factors, and whether they are truly underlying the development of the disease or simply existing concurrently with it, is not well understood.