Microcystis aeruginosa, cyanobacteria are frequent components of much fresh water marine ecosystem which often produce a hepatotoxin called microsytin. The toxin adversely affects the aquatic life forms, in the present study, its effect on Nain (Cirrhinus mrigala) was observed in biological parameters. In the fishes, the serum calcium after exposure to an acute concentration of microcystin for 96h, the values decreased significantly (P=0.05) to 9.9 mg 100 ml-1 compared to control (12.6 mg 100 ml-1). The acute exposure of microcystin also decreased the serum protein from 7.3 to 4.8. The fish showed hypercholesterolemia on exposure to acute and lethal concentrations of microcystin for 96h and both short (15-30 days) and long (45-90 days) term, respectively. The antioxidants, SOD activity did not change significantly in the liver, kidney or gills of fish that had been exposed to crushed cyanobacteria for 15 days at lethal concentration, however, the longer exposure (30 days) resulted in a significant increase (P=0.05) in the SOD activity in liver but induction in kidney and gills were not significant.