This study aimed to determine the lethal concentration 50% (LC 50 ) and behavioral, hematological, biochemical, histopathological, and genetic disorders of fipronil for Rhamdia quelen in acute toxicity tests for 96 h. To determine the LC 50 , 42 juveniles were distributed in groups of 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.8 mg L -1 of fipronil. For other evaluations 36 silver catfish were divided into control, 0.3 mg L -1 , and 0.4 mg L -1 groups. Blood, kidneys, livers, and gills were collected for evaluation. The LC 50 calculated with probittype regression was 0.811 mg L -1 at 48 h of study. There was total mortality of 0.8 mg L -1 , reduction in hematocrit values, and increase in liver enzymes. Liver samples showed cytoplasmic vacuolization and cellular degeneration, among other changes. Gills presented vascular congestion, complete fusion of secondary lamellae, and epithelial cell hypertrophy. In the kidneys, changes such as Bowman's capsule clearance, tubular degeneration, and glomerular capillary dilatation, among others, were common. Erythrocytes showed morphological alterations without increased micronucleus development. Fipronil induces a clinical condition of anemia, alterations in liver enzyme levels, nuclear erythrocyte changes, and liver, gill and kidney damage in the silver catfish.
