During a study of myxosporean parasites of cultivated freshwater fish, a new myxosporean species, Henneguya pellucida n. sp., was discovered. Of the 120 Piaractus mesopotamicus sampled, only 10 specimens (8.3 %) were infected. Yellow, round plasmodia measuring 0.5-3 mm were found in the serous membrane of the visceral cavity and in the tunica externa of the swim bladder. Sporogenesis was asynchronous, with the earliest developmental stages aligned prevailingly along the endoplasmic periphery and mature spores in the central zone. The mature spores were pear shaped (total length: 33.3 ± 1.5 μm, mean ± SD; width: 4.1 ± 0.4 μm; body length: 11.4 ± 0.3 μm; caudal process length: 24.1 ± 1.5 μm). The polar capsules were elongated (length: 4.0 ± 0.4 μm; width: 1.6 ± 0.2 μm). The development of the parasite in the swim bladder produced thickening of the tunica externa and a granulomatous reaction. 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