Common carp (Cyprinus carpio) and bighead carp (Aristichthys nobilis) were exposed to azimsulfuron and metsulfuron-methyl (50, 100 and 200 mg L -1 ). These herbicides are used in rice crop in Southern Brazil. Fishes survived to all tested concentrations of both herbicides and showed normal feeding and swimming behavior. Azimsulfuron inhibits significantly acetylcholinesterase (AChE) in brain and muscle of both species, and metsulfuron-methyl increase AChE activity in brain and inhibits in muscle. The present study showed that azimsulfuron and metsulfuron-methyl did not affect C. carpio and A. nobilis behaviors (feeding and swimming), but inhibited AChE activity in brain and muscle tissues of these species.