Marine parasitic nematodes are nearly as numerous as those of terrestrial and freshwater habitats; though their biodiversity is rather different. Parasites of land vertebrates, insects and soil invertebrates appear to be a large group of nematodes closely related to different groups of free-living nematodes hence they came to parasitism repeatedly from various origins. On the other hand marine parasitic nematodes comprise mostly Spirurida + Ascaridida complex of species associated with marine vertebrate hosts. This group originates in coastal region of the ocean and has been coevolving with ancient forms of marine fish. The few species beyond the Spirurida + Ascaridida complex are either considered as relatives of terrestrial parasites or their placement in nematode taxonomy is uncertain. Embryology and comparative anatomy data is not enough to clarify the possible origin of marine parasites. We obtained LSU rDNA sequences of two marine parasitic species from White Sea intertidal zone, Nematimermis enoplivora and Trophomera sp. Phylogenetic analysis have shown that N. enoplivora belongs to Mermithidae therefore it turned to marine parasite being a well developed terrestrial insect parasite. Thus we obtained the first forcible evidence of terrestrial parasite converting to marine apart from Spirurida + Ascaridida complex. Analysis of Trophomera sp. Sequences have shown that Benthimermithidae is segregated from any other families of nematodes but is a remote relative to both freshwater and marine Leptolaimida and Plectida. So we first found the placement of Benthimermithidae in nematode taxonomy and suppose this parasitic group originated in the sea.