Cypermethrin is one of the most commonly used pesticides. In this study, the effects of cypermethrin on serum biochemistry and liver histology of Anabas testudineus were investigated. The fish have been exposed to sub-lethal concentrations of cypermethrin for the 7th, 14th, and 21st days and one control was considered. AST, ALT, and ALP showed concentration- and days-dependent increases in all experimental groups. Bilirubin levels increased significantly (p<0.05) in cypermethrin groups. No statistically significant difference in bilirubin levels was observed between the concentrations of 0.015 and 0.030 mg L-1 on days 7th and 14th. Protein levels decreased in response to cypermethrin on all days when compared to controls. Statistically significant differences in protein levels weren’t observed between all concentrations on days 7th and 14th and between concentrations of 0.015 and 0.030 mg L-1 on days 21st. Light microscopy revealed hepatocyte hypertrophy, sinusoidal dilation, granular degeneration, congestion, pycnosis, and focal necrosis in the liver. AST ALT, ALP, bilirubin, protein levels, and histopathology can be used as possible markers for biological monitoring and chemical risk assessment in aquatic organisms. Manuscript profile