This article describes the morphological features and occurrence of Demodex ponderosus n. sp. on brown rats, Rattus norvegicus (Berkenhout), in Poland. D. ponderosus n. sp. is a small demodecid, adults average 152 µm in length, and are oval in shape. They were found in the skin of nearly hairless regions, tail and paws, of rats. Adult mites exhibit sexual dimorphism in terms of body size and proportions. The study also compared the features of the new species with those of the four species of Demodex described previously from this host. Demodex spp. were observed in 100% of the rats examined; however, despite high infestation levels, no symptoms of parasitosis were observed in the hosts.