The biology of the ascarid nematodes has been discussed in the context of their important economic role in farm animals, pet animals and zoo animals with special attention to carnivores and primates. In farm animals, infection with the most common roundworm of horses (Parascaris equorum) and swine (Ascaris suum) depend on many factors such as environmental conditions (larval development in the egg and egg survival), age of the host, breed, husbandry system, hygiene and treatment schedule. The monoxenic ascarids Toxocara canis and T. cati are the most important nematodes in carnivorous animals (dogs, cats, foxes) and carnivores in the zoo. In the period of March 2000 till March 2001, 57.1% of examined representatives of Felidae, Ursidae and Canidae in the Zoological Garden of Wrocław were found to be infected with ascarids (T. canis, T. cati, Toxascaris leonina). The prevalence of T. canis in the Canidae was 66.7%, of T. cati in the Felidae was 14.3%, while 57.1% of the Felidae carried T. leonina infection. Ascaris lumbricoides, a typical parasite of primates, was found in some gorillas, chimpanzees and orang-utans during parasitological survey.