Acta Protozool. (2005) 44: 377 - 384 Ultrastrutural Morphology of Myxobolus testicularis sp. n., Parasite of the Testis of Hemiodopsis microlepis (Teleostei: Hemiodontidae) from the NE of Brazil Jessica TAJDARI 1 , Edilson MATOS 2 , Ivete MENDONÇA 3 and Carlos AZEVEDO 1,4 1 Centre for Marine and Environmental Research, University of Porto (CIIMAR/UP), Porto, Portugal; 2 Carlos Azevedo Research Laboratory, Federal Rural University of the Amazonia, Belém (Pará), Brazil; 3 Laboratory of Animal Health, Federal University of Piauí (LASAN/UFPi), Teresina (Piauí), Brazil; 4 Department of Cell Biology, Institute of Biomedical Sciences, University of Porto (ICBAS/UP), Porto, Portugal Summary. Myxobolus testicularis sp. n. was found within cyst-like structures in the testis of the freshwater fish Hemiodopsis microlepis Kner, 1859 (Teleostei, Hemiodontidae) in close contact with the testis stroma. The fish were collected in the Poty River, near the city of Teresina (State of Piauí, Brazil). Large spherical to ellipsoidal plasmodia up to 0.5 mm were filled with disporic pansporoblasts and mature spores. The plasmodial wall presented a sinuous outline surrounded by several fibroblasts and a continuous layer of numerous collagen fibrils. The plasmodia contained different proliferative stages. Early sporogenic cells were present in the cortical zone. Immature and mature spores were located more internally. Ellipsoidal spores measured 8.6 ± 0.5 μm (n = 30) length, 7.2 ± 0.5 μm (n = 15) width, and 2.7 ± 0.4 μm (n = 12) thickness. The anterior end of the spores contained two equal pyriform polar capsules measuring 3.5 ± 0.3 μm (n = 15) length, 1.7 ± 0.4 μm (n = 10) width, each with an isofilar polar filament with 5-6 turns obliquely to the longitudinal axis. The wall of the polar capsule was filled with a hyaline substance contrasting with the very dense internal matrix. The spores consisted of two equal valves forming the wall. The spore wall was surrounded by a dense and uniform layer with variable thickness (up to 0.6 μm) consisting of a complicated network of anastomosed microfibrils closely adhered to the wall. More externally, in the plasmodial matrix, the fibrils formed a looser network. Based on the ultrastructural morphology and specificity to the host species, we propose the creation of a new species, named Myxobolus testicularis sp. n. Key words: fish parasite, Myxobolus testicularis sp. n., Myxozoa, ultrastructure. INTRODUCTION Numerous descriptions of myxosporeans have been reported in fish from different geographic areas (Landsberg and Lom 1991, Lom and Dyková 1992). Myxozoa Bütschli, 1882 comprises more than 1200 available species commonly found in fish (Lom and Dyková 1992). Among them, the genus Myxobolus Bütschli, 1882, with 744 species described, is the largest group of the Myxobolidae family (Eiras et al. 2005), and has been reported as an important pathogen in freshwa- ter fishes (Kent et al. 2001). Although considerable information has already been collected on South America myxozoans, the available descriptions are based, almost exclusively, on light mi- croscopical records and diagrammatic drawings of the spores (Aragão 1919, Nemeczek 1926, Penido 1927, Address for correspondence: Carlos Azevedo, Department of Cell Biology, Institute of Biomedical Sciences, University of Porto (ICBAS/UP), Lg. Abel Salazar no. 2, 4099-003 Porto, Portu- gal; Fax: + 351 22 206 22 32/33; E-mail: azevedoc@icbas.up.pt