Introduction The Demodex parasites were the first of the Demodecidae family (Acari, Trombidiformes) discovered by scientists and to date have been thoroughly researched. The canine demodecosis they cause is the most frequently occurring skin diseases in veterinary medicine. Even with numerous communications and studies on canine demodi- cosis, its symptoms, and methods of treatments, knowledge of etiological factors of this type of parasitosis remains incomplete. For over a hundred years, any reported case has been tied to one species only: viz. Demodex canis [1]. Miscellaneous symptoms, the course of the disease, and problems with treatment have suggested the existence of a more complex etiology. Even though D. canis has been described in detail by many authors [1-5], only recently has a second species of Demodex mites been identified: Demodex injai [6]. A number of communications on infes- tations caused by the so-called short-bodied form, regarded by some authors as a separate species, Demodex cornei [7], described later in studies that applied scanning microscopes [8-10]. However, the descriptions of these species remain insufficient – D. injai has been described based on homoge- nous material from the U.S., while D. cornei descriptions contain incomplete measurements and are based on a small sample of specimens to boot. No description of the juvenile stage has been provided for this species. The basic diagnostic methods used in detecting mites when demodicosis is suspected are based on microscopic analysis of skin scrapings. The disease is confirmed based on detection and identification of Demodex spp. It seems relevant, then, to find marker features in terms of morpho- logical details or body proportions that would facilitate dis- tinguishing species. Experimental Procedures The material comprised skin segments collected from 25 dogs, of which 20 had no symptoms of demodicosis and 5 had localized symptoms. The skin samples were taken from dead dogs in Gdańsk Pomerania between 2003 and Polish J. of Environ. Stud. Vol. 20, No. 3 (2011), 565-569 Original Research Diversity of Three Species of the Genus Demodex (Acari, Demodecidae) Parasitizing Dogs in Poland Joanna N. Izdebska*, Sławomira Fryderyk Laboratory of Parasitology and General Zoology, Department of Invertebrate Zoology, University of Gdańsk, Piłsudskiego 46, 81-378 Gdynia, Poland Received: 19 July 2010 Accepted: 4 January 2011 Abstract The occurrence of hair follicle mites (Acari, Demodecidae) was investigated in dogs from Gdańsk Pomerania, Poland. The three species of demodectic mites (Demodex canis, D. cornei, and D. injai) specific to dogs not only differ morphologically, but are also found on different areas of the hosts’ skin. Parasitic adap- tations involve size and shape of the body, which is elongated and worm-like, providing a specific adaptation to life in hair follicles, in various glands and their ducts or epidermis. The differentiation in structure, biology, and microhabitats of these mites may be associated with the occurrence in dogs of forms of demodecosis with different symptoms and courses. Keywords: Demodex, dog, demodecosis, parasites, infestation *e-mail: biojni@ug.edu.pl