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 adaptations 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.