SUMMARY Two wild Helianthus species native to North America have been naturalized in Argentina, H. annuus ssp. annuus and H. petiolaris. They grow as adventitious overlapping about 50% of the crop area. Hybridization and introgression between these wild species and sunflower have important biological and practical consequences, the former including homoploid hybrid species formation, and the latter including a possibility of transgene spreading from genetically modified (GM) sunflower cultivars to wild or weedy populations. Wild populations were screened for isozyme and morphological variation. Intermediate plants were found in several locations and subjected to progeny tests. Variability among progeny of each one was compared with the variability in wild accessions of both species. Segregation for phenotypic traits, intermediate phenology and low fertility levels were found in most progenies, accounting for the hybrid origin of their maternal plants. Attempts to quantify gene flow included screening of progenies from H. petiolaris populations growing near sunflower crops. Hybrid plants were recognized by morphological traits and reduced fertility. Overall hybridization was about 1%. Hybrid progeny on wild H. annuus plants were identified by a crop isozyme marker. A mean frequency of 7% hybridization was found. These results confirm that gene flow occurs among crop and wild Helianthus species, and it concerns crop management and environmental impact if release of GM sunflower cultivars is to be authorized in Argentina.