After a decade of transgenic crop production, the dynamics of crop gene introgression into populations of wild relatives remain unclear. Taking an ecological genetics approach to investigating fitness in crop-wild hybrid zones, we uncovered both conditions and characteristics that may promote introgression of crop alleles into wild populations. We compared the fitness of diverse crop-wild hybrid genotypes relative to wild Helianthus annuus under one benign and three stressful agricultural environments. Whereas relative fitness of crop-wild hybrids averaged 0.25 under benign conditions, with herbicide application or competition it reached 0.45. The variability of relative hybrid fitness also increased under more stressful conditions; in some instances, hybrid fitness matched wild fitness (i.e., relative fitness ≈ 1). Thus, wild populations under agronomic stress may be more susceptible to introgression. Although ‘domestication’ traits are typically considered unlikely to persist in wild populations, we found some (e.g., rapid growth and early flowering) that may enhance hybrid and wild fitness, especially in stressful environments. Rigorous assessment of how particular genotypes, phenotypes, and environments affect introgression will improve risk assessment for transgenic crops.