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Gastromermis cordobensis n. sp (Nematoda: Mermithidae) a parasite of larvae of the blackfly Simulium lahillei Paterson & Shannon (Diptera: Simuliidae) in Argentina, is described. Diagnostic characters of this species include a mouth ventrlly shifted; six cephalic papillae; eigh hypodermal chords; small and pear shaped amphids; a long and S-shaped vagina; a singl spicule, which is long, has non-uniform walls, and a tip with sculpture; three rows of genital papillae, the middle one with 18 pre-anal and 10 post-anal papillae, the lateral rows have 36 papillae each; oval eggs; and post-parasitic juveniles with long thin tails. Pre-parasitic and parasitic juveniles are included in the description.

Two new species of mermithids (Nematoda: Mermithidae), Gastromermis doloresi n. sp. a parasite of Simulium wolffhuegeli Roubaud and Hydromermis doloresi n. sp. a parasite of S. jujuyense (Paterson & Shannon) from Córdoba, Argentina, are described and illustrated. G. doloresi n. sp. is characterized by having medium sized, pear-shaped and oval amphids, eight hypodermal chords all around the body, cylindrical S-shaped vagina, single long spicule 528 [mi]m, whith teh sculpture tip, three rows of genital papillae in which the middle row contains 17 pre-anal and 9 post-anal papillae, and lateral rows contain 18 papillae. Hydromermis doloresi n. sp. can be distinghished by the following: medium sized, pear-shaped amphids, eight hypodermal chords, a cylindrical, S-shaped vagina, one short curved spicule with sculpture and genital papillae in three rows. The medium ventral row of papillae has 4 double and 9 single pre-anal papillae and 3 double and 6 single port-anal papillae, lateral rows have 18 single papillae each.

Two new species of the Mermithidae (Nematoda) are described from the Japanese blackfly Simulium japonicum Matsumura. They are the first representatives of Gastromermis and Isomermis from Japan. Gastromermis mesostoma n. sp. is characterized by a terminal or near terminal mouth, 6 hypodermal cords and the absence of cloacal or vulvar lips. Isomermis bipapillatus n. sp. is characterized by the small size of the spicules, their curved shape and by a pair of terminal hypodermal protuberances in the tail.

Twenty-four described mermithids are listed as known members of the Canadian fauna. Five of these are redescribed, and eight new species are described; namely Capitomermis micropeos, Gastromermis plethelos, Gastromermis leptopeos, Gastromermis pachopeos, Hydromermis macrospiculatus, Neomesomermis leptoderos, Neomesomermis welchi, and Neomesomermis yukonensis. A taxonomic key is provided to separate the 10 genera to which the 24 species belong. Keys identify the species of the genera Gastromermis, Hydromermis, and Neomesomermis. Morphology of the male head and genitalia are critically analysed and consistent male characteristics incorporated into four keys.

Gastromermis metae sp.n., a parasite of Simulium equinum and S. ornatum (Diptera: Simuliidae) is described and illustrated. The morphological variation in this species led to an evaluation of diagnostic characters currently used in mermithid taxonomy and an assessment of species in the genus Gastromermis. Ninety-five out of 99 published descriptions were examined: 25 were adequate, the remaining 70 are species inquirendae.

>Gastromermis boophthorae is distinguished from related species by its longer spicule, its characteristic lateral cord, numerous and small cells in the head region, and longer proximal reflex of the vagina. This species is divided into six varieties that are based mainly on differences occurring in the free-living postparasitic stages. It is recognized that the term “variety” is seldom used in nematology and is unprotected nomenclatorially, but its use here seems justified to designate infraspecific populations that appear at present to be sympatric and anatomically discontinuous, and that can only be recognized as immature nematodes. Six species of blackflies were found to be hosts and infection is reported from May until August in the Leningrad region. Percent infection ranged from 1 to 10 but reached 100% in one sample. Synchronization of host and parasite development is probably important in determining incidence of parasitism.

Caistromermis viridis and Isomermis wisconsinensis are described from larvae of Simulium vittatum Zett. in Wisconsin, and Mesomermis flumenalis from larvae of Simutlium vennstum Say in Ontario. No species of Gastromermis or Isomermis have previously been known from North America. The free-living and parasitic juveniles of all three species, as well as the adults, are described and illustrated.

Gastromermis deltensis n. sp., Hydromermis conopophaga Poinar, 1968, and Hydromermis palustris n. sp., parasites of chironomids (Diptera) in the Delta Marsh, Manitoba, Canada, are described and illustrated. The three species showed patterns of morphological variation, the most common being variation in ventral displacement of the mouth. This led to a reconsideration of the genera Gastromermis and Hydromermis. Spicule length is proposed as the differential characteristic between the two genera. Species placed to date in Gastromermis are reassessed on this basis.