The detection of mycotoxins in feeds and their ingredients in aquaculture gained prominence due to losses caused in production and animal health, mainly the occurrence of aflatoxin (AFB 1 ). The aim of this study was to evaluate the effects of AFB 1 on the performance of tambaqui fingerlings (Colossoma macropomum). Four hundred tambaqui were used. Four different treatments were evaluated: treatment T1, considered as the control treatment (CT) with 3.84 lg kg -1 ; treatment T2, treatment T3 and treatment T4 with 500, 1000 and 2000 lg kg -1 of AFB 1 , respectively. The AFB 1 of the samples (muscle, liver and kidney) was detected by high-pressure liquid chromatography. Four fingerlings from each treatment for histological analysis were examined. Moreover, the performance parameters (weight gain, feed conversion and feed intake) were studied. The levels of toxins used in T2, T3 and T4 represent a reduction in the growth of 14%, 35% and 45%, respectively. The T3 and T4 showed the lowest weight gain (78%) and the worst feed conversion. Aflatoxin B 1 in muscle (3.28 lg kg -1 ) and kidneys (8.8 lg kg -1 ) in the T3, as well as liver (4.4 lg kg -1 ) and kidney (4.08 lg kg -1 ) in T4, was detected. Histopathological changes in liver and kidney tissues of fingerlings were more pronounced in T3 and T4. Fingerlings that consume feed contaminated with AFB 1 in concentrations higher than 500 lg kg -1 present decreases in growth, reduction in weight gain and feed intake with increased feed conversion. The consumption of feed contaminated with 1000 and 2000 lg kg -1 of AFB 1 caused severe deterioration in the hepatic and renal tissues.