High rates of hybridization and introgression have been reported between the cultivated sunflower and its wild progenitor (both Helianthus annuus), raising concerns that neutral or favorable transgenes might escape and persist in wild H. annuus populations. However, little consideration has been given to the possibility that other wild sunflower species may hybridize with the cultivated sunflower. Here, we estimate the frequency of hybridization and introgression between the cultivated sunflower and Helianthus petiolaris, a closely related wild species whose range almost completely overlaps that of the cultivated sunflower in the central and northern plains and which hybridizes extensively with wild H. annuus. Assays of 21 cultivar-specific AFLP (amplified fragment length polymorphism) markers in 159 individuals from four sympatric populations of H. petiolaris revealed low rates of introgression, with average marker frequencies per population ranging from 0.006 to 0.026. Twenty-seven individuals had a hybrid ancestry, and all hybrids appeared to represent later-generation backcrosses. There was significant heterogeneity in rates of introgression for different markers, and eight of the 21 markers failed to introgress into any of the four populations. This and other evidence indicate that the H. petiolaris genome may be differentially permeable to introgression. These data indicate that (1) beneficial and neutral transgenes will take much longer to move into H. petiolaris populations than into wild H. annuus; (2) the risk of escape may depend on the genomic location of the transgene; and (3) escape is likely to be sporadic, occurring in some populations and not others and at different times. Thus, risk assessment in wild H. annuus is of more immediate concern than in H. petiolaris.