Weedy sunflower is an invasive plant on the territory of the Republic of Serbia, which causes high yield losses in many crops. During the harvesting of the sunflower crops the dispersal of the seeds occurs, and as a result- the volunteer plants appear next year. Weedy sunflowers originate from volunteer plants that live through a longer period in one place. Spontaneous hybridization of weedy sunflower with other sunflower forms makes them more aggressive. If the volunteer plants originate from the hybrids tolerant to ALS inhibiting herbicides, they can be the carriers of herbicide tolerance genes and thus will not be sensitive to these herbicides. The exchange of the genetic material also enables the transfer of the ALS (AHAS) gene (responsible for the tolerance to the ALS inhibiting herbicides) to the progeny. In this study we have examined the spontaneous hybridization between different sunflower forms (volunteer sunflowers, weedy sunflowers, susceptible and tolerant sunflower hybrids to ALS inhibiting herbicides) in field conditions during three years. The progeny (F1 generation), which was assumed to possess the ALS gene, was tested with the application of the recommended doses of the Express (a.i. tribenuron-methyl) and Pulsar 40 herbicides (a.i. imazamox). The significant percent of the progeny of different forms of sunflowers, survived the herbicide treatment (6-31%). Molecular analysis of the ALS gene sequence in weedy sunflower progeny confirmed gene transfer in two cases at a distance of 30 and 120 m from the gene donor, i.e. tolerant hybrid Sumo 1 PR.