Diclofop-methyl is being increasingly used as an herbicide under intensive cultivation to control weeds in wheat. The bioconcentration of diclofop-methyl in two different groups of freshwater fish (Oreochromis niloticus) was investigated after 28 days of exposure. Low and high concentrations applied to two different fish groups were 1/10 (0.19 mg L-1) and 1/3 (0.63 mg L-1) of the determined 96 h-LC50, respectively. The bioconcentration factors for whole fish were 7.05 and 9.21 for fish exposed to lower and higher concentrations, respectively; while the respective bioconcentration factors for muscle tissue were 1.61 and 2.15 LC for fish exposed to lower and higher concentrations, respectively. Diclofop-acid was the main metabolite during the exposure period, ranging concentrations from 5 to 8% of the parent diclofop-methyl concentration.