Mercury is a xenobiotic contaminant often found in natural ecosystems. It is relevant for public and environmental health because of the existing correlation between its content in water sources and mercury bioaccumulation in living organisms. This work assessed the immune and histopathological effects of exposure to sublethal concentrations of mercury chloride (HgCl2) in Pacu (Piaractus brachypomus). Pacu fingerlings weighing 10 ± 2.1 g were distributed in constantly aerated tanks with no filter. The experimental period was 18 days. A negative control group and four Hg levels were used based on the tenth of LC50 for Pacu. Six blood samples were taken on days 1, 2, 4, 7, 12 and 18 to measure respiratory burst and bactericidal activity of the plasma. The hepatosomatic index was calculated and samples were taken for histopathological examination. Increased respiratory burst (oxidative stress) was observed in animals exposed to HgCl2 in a concentration-dependent manner. This effect was more pronounced at day 12 of exposure. Hepatosomatic index (HSI) values showed no significant differences. Animals exposed to HgCl2 showed low bactericidal activity of plasma. Histopathological changes such as hyperplasia, aneurysms and synechiae were found in gills, while hyaline inclusions were observed in liver and melanomacrophage centers in kidney. Pacu fingerlings exposed to sublethal doses of HgCl2 had a significant increase in oxidative stress and changes in plasma bactericidal activity in addition to pathological changes in the gills, hepatic and renal tissues