Based on light and scanning electron microscopical studies, two new species of nematode parasites are described from freshwater fishes in Thailand: Orientatractis mekongensis n. sp. (Atractidae) from the intestine of Pangasius bocourti Sauvage (type-host) and Helicophagus leptorhynchus Ng & Kottelat (both Pangasiidae, Siluriformes), and Neosynodontisia suratthaniensis n. sp. (Pharyngodonidae) from the intestine of Labiobarbus siamensis (Sauvage) (Cyprinidae, Cypriniformes), for which a new genus Neosynodontisia n. g. is established. Orientatractis mekongensis is mainly characterised by the number and distribution of caudal papillae (2 preanal, 1 adanal and 5 postanal pairs), the length of the left spicule (306-384 µm) and large body sizes (length of males and gravid females 5.4-6.7 mm and 7.8-9.0 mm, respectively). Neosynodontisia differs from other pharyngodonid genera with representatives parasitic in fishes not only by some morphological features (mouth withdrawn into the cephalic end with inflated cuticle, structure of the male caudal end, filamented eggs), but mainly by the occurrence of males inside the body of females. A key to the genera of the Pharyngodonidae with representatives parasitising fishes is provided.

Department of Fisheries Faculty of Agriculture and Technology Nakhon Phanom University Nakhom Phanom 48000 Thailand

Department of Fishery and Coastal Resources Faculty of Science and Industrial Technology Prince of Songkla University Surat Thani Campus Surat Thani 84000 Thailand

Institute of Parasitology Biology Centre of the Czech Academy of Sciences Branišovská 31 370 05 České Budějovice Czech Republic

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