Weedy sunflower Helianthus annuus L. (WS) can become troublesome for cultivated sunflower given their genetic similarity which allows gene flow from weed-to-crop and from crop-to-weed. Also, it causes yield losses in different spring-sown crops. Despite this, there is still no data about its response to herbicides. Field experiments were conducted to quantify the effects of nicosulfuron on WS populations. Plants of three populations (R1-WS and R2-WS = presumably resistant and S-WS = susceptible) were treated with 40 g nicosulfuron ha-1 at the 2–4 true-leaf growth stage. Vegetative parameters (plant height, fresh weight, leaf area) and relative chlorophyll content (RCC) were measured 30 days after herbicide application. Generative parameters (number of heads plant-1, head diameter, number of seeds plant-1) were measured at maturity. After harvesting, seed germination of collected seeds was studied in Petri dishes at 25°C. Nicosulfuron was found to have a significant effect on vegetative and generative production of WS plants. The effect on RCC and postharvest seed germination was not so prominent. All this effects depended on the population, which is probably a result of different history of herbicide application and possible development of resistance in the populations R1-WS and R2-WS.