Nematodes are one of the most numerous and widespread groups of invertebrates, inhabiting sea, freshwater and soil, and parasitizing plants and animals. The total number of nematodes worldwide is estimated by experts to be from 100 thousand to 1 million species, and a thorough study of the class is under way; with new species, genera, families and orders being described. This new information is leading to a general transfer of taxa within the class Nematoda as a whole (Malakhov, 1986), and particularly in certain ecological groups (Paramonov, 1962; Paramonov,-1964; Lorenzen, 1981; Gerlach, Riemann et al., 1974). At present there are two systems in widespread use for the class: one with two subclasses (Chitwood, 1933; Chitwood, 1937; Chitwood, 1958), the other with three subclasses (Andrassy, 1984). About 20000 nematode species have been described to date, including 2000 plant helminths, 5000 animal parasites, and 13000 free-living nematodes in sea, freshwater and soil. About 500 species of 200 genera in 10 orders are known to occur in freshwater (Kirianova & Krall', 1969; Tsalolikhin, 1983; Tsalolikhin, 1985; Chesunov, 1991; Eliava, 1994; Coninck, 1965; Goffart, 1959), and about 350 species of 130 genera have been recorded in freshwaters of the former Soviet Union (Gagarin, 1981; Gagarin, 1993; Tsalolikhin & Gagarin, 1972; Lemzina, 1982; Philipiev, 1937). In Ukraine, where nematodology has developed mainly as a part of helminthology, a total of about 1200 species have been recorded, including 750 parasiting plants and animals (Sigariova, 1972; Sharpilo et al., 1993), 200 sea species (Sergeyeva, 1974; Sergeyeva, 1982; Platonova, 1968), and nearly 120 species of 80 genera, 42 families and 7 orders in fresh and brackish waters (Gurvich, 1964; Gurvich, 1971; Gurvich, 1972; Dekhtiar, 1985; Dekhtiar, 1988; Dekhtiar, 1989; Ivaneha, 1976; Ivaneha, 1978; Paramonov, 1929; Philipiev, 1915; Micoletzky, 1917). Nematodes play an enormous part in natural ecosystems as decomposers due to their abundance, intensity of metabolism and short life cycle (Novikova, 1976; Striganova, 1983; Striganova, 1985; Caldwell & Bovee, 1975; Sohlenjus, 1980). Nematode species for which the ecologically optimum level has been determined are used in monitoring as reliable indicators (Dekhtiar, 1980; Dekhtiar, 1994; Bonger & Haar, 1990; Callahan et al., 1991). Free-living nematodes have not yet been fully studied in Ukraine, because soil, many freshwater ecosystems, and the coasts of the Black and Azov Seas have not been examined. It is likely that world's nematode fauna will be enlarged by the description of new species of free-living nematodes.