Ultrastructural analyses of fish-infecting myxosporean Henneguya piaractus that is found in the gill lamellae of the freshwater teleost Piaractus mesopotamicus (Characidae) and collected from the Paraguai River, Brazil were described. The parasite occurs within large whitish spherical to ellipsoidal polysporic cysts (up to 2.5 mm long) delimited by a layer of fibroblasts generally connected with some capillaries on the gill epithelium. No external morphological signs of disease were visible in the infected fishes. The tailed spores measured 61.5 ± 0.91 (60.2–62.6) μm in total length and ellipsoidal spore body 21.1 ± 0.62 (20.6–21.9) μm long, 6.7 ± 0.40 (6.2–7.3) μm wide and 2.5 ± 0.54 (2.0–3.1) μm thick. The spore wall was about 97 nm of thickness and consisted of a thin electron-dense exospore and a thick electron-lucent endospore with about 85 nm of thickness. The tailed spores were composed of two equal–sized shell valves adhering together along the straight suture line each having in continuity a equal caudal tapering tail measuring 40.5 ± 1.02 (38.7–43.1) μm in length. Two symmetric polar capsules measured 9.8 ± 0.28 (9.3–10.1) μm long and 1.9 ± 0.37 (1.4–2.4) μm wide, each having a polar filament with 10–11 (rarely 12) coils