Residues of ¹⁴C-fenitrothion in a model rice-fish ecosystem and field rice-fish ecosystem were studied. When equal amounts of the pesticide were applied, the extractable residues in brown rice (equivalent to 34.3 ± 1.9µg/kg fenitrothion) and rice stems and leaves (20.9 ± 1.5µg/kg) of the model rice-fish ecosystem were 10—15 times higher than that of the field rice-fish ecosystem (4.48 ± 0.13µg/kg and 1.27 ± 0.34µg/kg respectively). Residues in upper part of the soil (6.50 ± 0.1—8.10 ± 0.2µg/kg) and lower part of the soil (1.30 ± 0.1—1.50 ± 0.1µg/kg) of the model rice-fish ecosystem were 10—40 times higher than that of the field rice-fish ecosystem (0.17 ± 0.01µg / kg). The extractable residues in paddy water of the model ecosystem (0.30 ± 0.01µg/kg) were similar to that of the field ecosystem (0.20 ± 0.02µg / kg). When the fenitrothion was sprayed on the rice plants, residues in brown rice, fish body, soil and paddy water were lower than those when the pesticide was sprayed on the surface of the soil.