Phalaris minor populations (95 No.) were collected from 14 districts of Punjab. All these populations were not exposed to isoproturon for more than 15 years. Plants of each population/pot were planted and treated with the graded doses of isoproturon 75WP (234.5, 469, 938, 1876, 3752 g/ha) 30 days after sowing as per treatment. The mortality data was recorded and converted to morality percentage in relation to untreated control populations. Nonlinear regression analysis was used to determine the mean dose that caused mortality by 50% (GR50). Out of 95 Phalaris minor populations, isoproturon (IPU) at 938 g/ha provided more than 90% mortality in 39 populations and 70-90% mortality in 17 populations, respectively.