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A microsporidian infection due to Microspolidiun~ arthun.n sp., has been observed in Pangasius sutchi from South-East Asia. The parasite forms uninucleate spores averaging 3.1 X 2 1 pm, variable in shape, mostly rounded pyriform but often curved. The exospore has a pattern of mosaic-like small fields. Developmental stages were not observed. In heavy infections, large parasite masses destroyed parts of the musculature but host reaction was mostly limited to phagocytosis of spores Among vertebrates, fish constitute the class most extensively parasitised by microsporidia. Thus far, if the species recorded in Canning & Lom (1986) are supplemented by recent descriptions, a total of 81 named and 30 unnamed species are known, belonging to the genera Glugea, Heterosporis, Ichthyosporidium, Loma, Microgemma, Jirovecia, Pleistophora, Spraguea, Tetramicra, Thelohania and the collective group Microsporidium. The 2 species assigned to the genus Thelohania should probably be reclassified in some other genus, and single species of the genus Jirovecia may have been a parasite of the prey animals. Unnamed species constitue one thlrd of the total number of fish microsporidia. Species assigned to the collective group Microsporidium account for almost one fifth of the total number of species in fish. A proper generic assignment for these species is impossible because only mature spores and no developmental stages are known. Such is the situation of new species which we found during a short stay of the senior author at the University Pertanian Malaysia in Serdang. We publish its description to stimulate further research on the little-known microsporidia and other fish parasites in this region. Materials and methods. Pangasius sutchi were purchased from a pet shop in Serdang, state of Selangor in Malaysia. They had been imported froin Bangkok, Thailand. Squash preparations from all body E Inter-Research/Printed in F. R. Germany organs were examined and fresh spores were observed and photographed. Smears were treated with periodic acid-Schiff (PAS) reagent for polysaccharides and with Feulgen nuclear stain. Three heavily infected specimens of Pangasius sutchi were sampled for histology. Pieces of infected tissue were fixed in 10% buffered formalin processed for embedding in paraffin wax, sectioned at 5 pm and stained with haemotoxylin-eosin and Giemsa stains. Samples from 3 more fish were fixed in cold 2 % osmic acid in 0.1 M cacodylate buffer and embedded in Epon-Araldite. Ultrathin sections, double stained with uranyl acetate and lead citrate, were observed in a Philips 420 B electron microscope operated at 80 kV. Results and discussion. Twenty-five fish were examined. In addition to a Hexam~ta species found in one specimen and spores of a Myxobolus sp. in 4 specimens, infection due to a microsporidium was found in 9 specimens in the trunk muscles. In 4 fish with heavy infections white focl were not visible through the skin. Each focus consisted of masses of spores. The spores were of variable shape (Fig. l), mostly pyriform, often curved, comma-shaped, or ellipsoidal to cylindrical, and some were almost subspherical. The anterior end was always narrower than the posterior one. The size was 3.1 (2.3 to 3.8) X 2.1 (1.8 to 2.2) pm (n = 40). The posterior vacuole occupied one half to one third of the spore length, depending on the spore shape. The PASpositive polar cap appeared as a tiny dot and the Feulgen reaction showed a single nucleus. Electron microscopy revealed little of the spore contents except for 6 turns of the coiled isofilar polar tube (Fig. 2 ) . The exospore was raised into a mosaic of irregular fields separated by grooves which were rectangular in transverse section (Figs. 3 and 4).

Microsporidium arthuri n, sp., parasite of Pangasius sutchi (Pangasiidae, Siluroidea) in South-East Asia