Fenthion, an irreversible cholinesterase inhibitor, was used to study the role of the cholinergic system on the development of gizzard erosion. Fenthion increases the gizzard erosion score in a dose-dependent manner and this effect became significant at levels higher than .1 ppm (p less than .05). An inverse relationship between plasma cholinesterase activity and pesticide concentration was also observed at doses higher than 1 ppm (P less than .05). These results show the necessity to evaluate organophosphate pesticide levels during the selection of fish meals in poultry.