The inhibitory effects of four carbamate insecticides, methomyl, thiodicarb, carbofuran and carbosulfan, on acetylcholinesterase of male and female Carassius auratus were investigated in the laboratory. Kinetic constants, biomolecular rate constant (k(i)), carbamylation rate constant (k2) and decarbamylation rate constant (k3) were determined in vitro. The ratios of bimolecular rate constant (female/male) for AChE reacting with methomyl, thiodicarb, carbofuran and carbosulfan were 1.03, 2.44, 1.03 and 1.106, respectively. The k(i) of AChE for thiodicarb was significantly higher in female fish than in male fish (P<0.05). The ratios of carbamylation rate constant (female/male) for methomyl, thiodicarb, carbofuran and carbosulfan were 1.18, 4.29, 3.53, and 2.07, respectively. The k2 values of AChE for the above four carbamates were significantly higher in females than in males. The ratios of the decarbamylation rate constant (female/male) for methomyl, thiodicarb, carbofuran and carbosulfan were 1.02, 1.39, 1.06, and 1.98, respectively. Only for carbosulfan, the decarbamylation rate of AChE was significantly higher in the female than in the male, indicating that AChE of females inhibited by carbosulfan recovered more quickly than that of males. The above results suggest that the female in the sensitivity of AChE to carbamates was different from the male fish.