Paratenic hosts, especially of nematodes, play a major role in spreading many parasite species. The phenomenon of "paratenic parasitism" is illustrated by examples of development of Syngamus trachea and Proteocephalus longicollis. Moravec introduced a concept of "natural paratenic host" for P. longicollis infecting common sculpion, Cottus gobio, in Europe. According to Kennedy, paratenic host richness of Anguillicola crassus is an adaptation to new environmental conditions. New studies, involving laboratorial experiments, have shown that the nematodes parasitizing fishes, with a single intermediate host in their life cycles, acquired numerous paratenic hosts. It also evident that a paratenic host can be present or absent in the life cycles of related (congeneric) parasite species (Bothriocephalus gregarious and B. barbatus; Halipegus occidentalis and H. ovocaudatus; Dracunculus insignis and D. medinensis). Sometimes infective larvae of parasites are reported in accidental hosts, which do not occur in the food chain of the definitive host. Paratenic host is more an ecological than a physiological phenomenon in the transmission of helminths. Under extreme environmental conditions the transmission of parasites is faciliated mainly by paratenic hosts (e.g., Toxascaris leonina or Toxocara sp. occurring in zoological gardens where sanitation is rigorously observed).