An experiment was conduced on adult male and female Nile tilapia to investigate the effect of Methidathion (MD,is one of the more toxic organophosphates) MD at doses 3 \xg l"1 '10 jig l"1,20 jag r'on the microsomal of ethoxy-resrufin o-deethylase activity (EROD). The experimental fish was obtained from Manzala Lake from the areas suffering from pollution. Result revealed that activity of EROD in female livers jig/kg/ fresh liver was (3,33,25,17,12) for MD concentrations 3|igl_i; 1 Opgl"1 ;20jigl*!, Control and IC groups, respectively. Analysis of variance indicated that differences among the tested groups were not significant P