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The Systematics of trichostrongyloid nematodes of ruminants provides a foundation for diagnostics and responds to the need to identify eggs in feces, free-living larvae from pastures or fecal cultures and larval or adult nematodes collected from hosts. These needs are associated with diagnostic problems or research projects. Difficulties in identifying all developmental stages of trichostrongyloid nematodes of domestic ruminants still severely limit the effective diagnosis and control of these parasites. Phylogentetic hypotheses as the basis for predictive classifications have been developed only for the subfamilies of the Trichostrongylidae. This report briefly describes recent progress in the development of improved tools for identification, phylogenetic analyses and predictive classifications. It also describes future research needed on the identification and classification of trichostronglyoid nematode parasites of domestic ruminants. Nematodes included are species of the superfamily Trichostrongloidea known to be important pathogens of domestic ruminants. The information summarized is presented by nematode developmental stage and by taxonomic groups. Eggs: While eggs of some trichostrongyloid nematode parasites of ruminants can be readily identified to their genus (Nematodirus), and some to species (e.g. Nematodirus battus), most of the important pathogens (including the Ostertagiinae and Haemonchinae) cannot be identified morphologically even to family level. However, DNA technology has been developed for determining not only the presence of specific pathogens in eggs from fecal samples, but also for estimating the percentage of the total eggs that each pathogens comprises. This new method will make possible a rapid determination of which individual animals in a herd should be treated. Larvae: The most commonly used method for identifying infective larvae is time-consuming (several weeks), unreliable for estimating intensities of individual species as components of mixed populations and requires highly-trained specialists. Available identification keys for larvae are not well illustrated and need to be augmented. Adults: Recent advances in the identifications of adult trichostronglyoids and their systematics are organized by taxonomic group. General included are Ostertagia, Haemonchus, Cooperia, Trichostrongylus and Nematodirus. Recently, the first phylogenetic analysis of the Trichostrongylidae family established monophyly for the family. A similar analysis of the Molineidae is needed. Ostertagia: Several studies of polymorphism summarized the phenomenon and listed 19 polymorphic species of Osteragiinae supported of DNA differences within and among polymorphic species of Ostertagiinae supported earlier hypotheses that the species pairs represent polymorphic species. A phylogenetic analysis of the Ostertagiinae and generic concepts are needed. Haemonchus: A key to three species of Haemonchus provides, for the first time, morphological characteristics for the microscopical identifucation to species of individual adult nematodes of either sex. The Food and Drug Administration is now requiring that results of drug trials included identification of Haemonchus to species. Cooperia: Studies using random amplified polymorphic DNA methods showed a high degree of variation within and among C. oncophora/ C. surnabada. but supported a polymorphic relationship for the species pair. A phylogenetic analysis of the Cooperiinae is needed. Trichostronglys: Restriction Fragment Lenght Polymorphisms (RFLPs) of genomic DNA polymorphism. However, other studies demonstrated expected species-level differences between T. colubriformis and T. vitrinus using Random Amplified Polymorphic DNA (RAPD) methods. Sequences of the second Internal Transcribed Spacer Region (ITS-2) ribosomal repeat showed sequence differences of 1.3-7.6% among five well-defined species of Trichostrongylus. This provides a standard for species-level differences within the Trichostrogylidae. Nematodirus: This origin of N. battus in the British Isles is still a mystery. Recently, DNA studies have provided evidence that populations on both coasts of the United States originated from Canada. A phylogenetic study of Nematodirus is in progress. Modern systematic methods have not yet been applied to the development of classifications for all subfamilies and most genera of the Trichostrongyloidea. Additional factors complicating these problems are a lack of knowledge of the parasites of wild bovids and cervids, the international transport of wild and domestic hosts and environmental changes that may alter the parasite fauna in a modern farm.