This study was aimed at evaluating the effects of cadmium (Cd) on bioaccumulation, acetylcholinesterase (AChE) activities and histopathological alteration in African catfish (Clarias gariepinus) from a contaminated fish farm in Mae Sot District, Tak Province, western Thailand. Along with water and sediment samplings, fish samples were collected from the contaminated fish farm after two, four and six months of cultivation. The Cd accumulation in liver and muscle of C. gariepinus were measured. Moreover, the effects of Cd on AChE activities in brain, kidney, liver, and muscle were studied. The results showed that Cd concentrations in water of Mae Tao Stream and fish pond ranged 0.001-0.038 mg/l, while in the sediments ranged 0.367-21.250 mg/kg. Cadmium concentration in the liver of C. gariepinus was higher than in the muscle, i.e., 0.190 mg/kg (range 0.150-0.270 mg/kg) and 0.030 mg/kg (range 0.020-0.040 mg/kg), respectively. AChE activities showed significant increase in the brain and the muscle and decrease in the kidney and the liver. In addition, after six months of fish cultivation, AChE activities were found significantly decreased in the liver. Furthermore, histopathological alterations were observed in the gills, the kidney and the liver after exposure, i.e., loss of all mucus membranes of the gills, distortion of capillaries of glomerulus in kidneys and large vacuoles in liver tissues. These results are useful for biomarker of Cd contamination in the aquatic environment.