The objective of this study was to evaluate biochemical parameters and histopathology of liver in Matrinxã (Brycon cephalus) and Pacu (Piaractus mesopotamicus) fish chronically exposed to dietary aflatoxins. Fish feed was artificially contaminated with aflatoxins and the treatments were: Control – feed without toxin; Treatment A – feed + 10 μg aflatoxin B₁ (AFB₁)/kg; Treatment B – feed + 20 μg AFB₁/kg; and Treatment C – feed + 50 μg AFB₁/kg. Matrinxã and Pacu juvenile fish were placed in tanks for 180 days. Five experimental units per treatment were monthly sampled and submitted to blood collection and removal of hepatic tissue. Thus, twenty blood and liver samples for each species were collected monthly, adding up to 240 samples analysed. To verify biochemical changes, analyses included total proteins, albumin, globulins, aspartate aminotransferase (AST) and alkaline phosphatase (ALP). The hepatic tissue was examined microscopically and the slides presenting histopathological changes were photo-documented. There was effect of treatment (P<0.05) for AST and ALP in Matrinxã, while no effect (P>0.05) was observed in Pacu. A reduction (P<0.05) in AST and ALP values during the time of exposure was observed in all treatments for both species. Fatty degeneration and liver damage were observed for both species in treatments exposed to aflatoxins. Fatty degeneration in Pacu was noticed after 30 days of exposure, while in Matrinxã it was observed after 60 days. Disorganisation of the hepatocyte cord arrangement was also observed in those treatments exposed to aflatoxin, following 90 days of exposure in Matrinxã, and after 60 days in Pacu. Therefore, aflatoxins have little influence on biochemical parameters in the species evaluated. However, exposure to aflatoxins caused liver changes, such as cell death, fatty and hydropic degeneration, thus it could be concluded that both species are susceptible to the toxic effects of long-term exposure to dietary AFB₁.