An enzyme test for determining isomalathion (O,S-dimethyl-S-(1,2-dicarbethoxyethyl) phosphorodithioate) impurities in water-dispersible powders of malathion (WDP malathion) is described. The test is based on inhibition of acetylcholinesterase (EC 3.1.1.7) by isomalathion extracted from WDP malathion. The lower limit of detection of the test is 0.01% (w/w) isomalathion. For 18 samples of WDP malathion there was good correlation between the levels of isomalathion found using the enzyme test and those obtained by thin-layer chromatography.