Abstract: To evaluate the effects of dietary aflatoxin B₁ (AFB₁) on growth performance, feed efficiency, and histological changes in juvenile red swamp crayfish (Procambarus clarkii), crayfish with initial body weight of (0.382±0.005) g were fed with four diets containing 0, 10, 100 and 1000 μg/kg AFB₁ for a 42-day trial. Significant lower survival rate (SR), feeding rate (FR), final body weight (FBW), specific growth rate (SGR) and feed efficiency (FE) were observed in 100 μg/kg and 1000 μg/kg AFB₁ group, while no significant differences were found in 10 μg/kg group compared to control group. There were no significant differences in alkaline phosphatase (AKP), alanine aminotransferase (ALT), aspartate, aminotransferase (AST), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione S-transferase (GST) and malondialdehyde (MDA) in hepatopancreas between the control and 10 μg/kg AFB₁ group, while significant decrease in superoxide dismutase (SOD) was observed in 10 μg/kg AFB₁ group. AFB₁ above 100 μg/kg significantly impacted activities of the aforementioned hepatopancreas enzymes. Slight histological changes were identified in hepatopancreas of 10 μg/kg AFB₁ group. However, severe lesions of hepatopancreas were found in 100 μg/kg and 1000 μg/kg AFB₁ group, which showed decreased numbers of R-cells and increased numbers of B-cells. The ultrastructural results showed that with the increase of AFB₁ concentration, hepatopancreas demonstrated swelling mitochondria, expanded rough endoplasmic reticulum, increased cytoplasmic vesicles and larger lipid droplets. No AFB₁ residues were detected in whole body when crayfish were fed with AFB₁ up to 100 μg/kg, however, tiny dose (1.65 μg/kg) of AFB₁ were detected in the group with 1000 μg/kg AFB₁, which was below the safety limitation of FDA. In summary, dietary AFB₁ above 10 μg/kg had an adverse effect on juvenile crayfish, which indicated that red swamp crayfish was a sensitive species to AFB₁.