Las características macroscópicas e histopatológicas de la myxosporidiosis branquial causada por Henneguyasp. Fueron descriptas en 44 ejemplares de Serrasalmus sp. De la región nordeste argentina. Los quistes de myxosporídios más grandes (1,5-3,0mm) se localizaron en las láminas primarias, y tenían tabiques internos con vasos sanguíneos. Los quistes más pequeños (25-75 m) estaban en las laminillas secundarias, rodeados de una delgada cápsula. Se observó reacción inflamatoria, infiltrado mononuclear, hiperplasia de células epiteliales y fusión de laminillas secundarias. Microscopic and histopathological characteristics of the myxosporidiosis on gills caused by Henneguya sp. were described in 44 specimens of Serrasalmus sp. from the argetine northeast. The largest myxosporidian cysts (1, 5-3, 0 mm) were localized in the primary filament, and had internhal trabeculae with blood-vessels. The smaller cysts (25-75 µm) grew within the secondary lamellae, and were found to be covered by a thin capsulae. Inflamatory reaction, mononuclear infiltration, hyperplasia of epithelial cells and secondary lamellae fusion, were here also observed.