Weedy sunflower has become a problem worldwide, giving significant yield losses in sunflower and soybean fields even at low densities, decreasing their oil quality and allowing gene flow among crops and weeds. Its evolution differs among countries due to availability of wild forms. The problem is not only weedeness but also herbicide resistance in the Republic of Serbia. Three weedy sunflower populations from Serbia were studied: RWS1 and RWS2, which were presumably ALS herbicide resistant and SWS which is susceptible. Plant height, fresh weight, leaf area, relative chlorophyll content, fecundity and percentage of germination, length and weight of seedlings with and without nicosulfuron application were recorded. The most frequently RWS1 and RWS2 populations were of better ecological fitness than the SWS population under the conditions with and without nicosulfuron application. The number of seed produced was higher in RWS1 without herbicide application and RWS2 with nicosulfuron application. It was concluded that the differences in the level of herbicide-resistance could result in different fitness level of weedy sunflower populations which could promote the invasiveness of these populations in landscape.