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Trichinella spiralis is a parasitic nematode causing trichinellosis, a serious disease, and Caenorhabditis elegans is a free-living nematode, which is used as a model in parasitological studies. High levels of thymidylate synthase (EC 2.1.1.45; ThyA) and certain other enzymes involved in thymidylate biosynthesis were found throughout T. spiralis and C. elegans developmental cycles, including developmentally arrested forms, that is, T. spiralis muscle larva and C. elegans dauer larva. As ThyA activity is characteristic for cells that left the G0 phase of the cell cycle, an exceptional regulation of the cell cycle in nematodes is suggested, manifested by a global cell cycle arrest in developmentally arrested larvae of the two species. ThyA immunolocalization during development of T. spiralis and C. elegans revealed the presence of high enzyme levels not only in the developing embryos, where it was expected, but also in gonad primordia, egg and sperm cells, nerve ring and secretory cells, opening to T. spiralis esophagus and C. elegans pharynx, where it may point to those cell populations remaining cell cycle arrested.