The movement of pollen between crop and wild sunflowers (both Helianthus annuus) has led to concerns about the possible introduction of crop transgenes into wild populations. The persistence of crop traits in wild populations will depend in part on the relative fitness of crop-wild hybrid vs. wild plants. Using seeds from two large experimental field plots, we found that seeds produced by crop-wild plants were twice the size of wild seeds and differed in coloration. Head diameter, date of flowering, identity of mother plant, and levels of predispersal predation explained some variation in mean seed size. We hypothesized that postdispersal vertebrate seed predation would be affected by seed size, with hybrid seeds preferentially eaten. In each of three field trials, significantly more hybrid seeds were eaten (62% of hybrid seed; 42% of wild seed). Within the category of wild seeds, larger seeds were preferentially eaten; however among hybrid seeds, predation was not significantly related to seed size. In this study, differential predation thus reduces hybrid fitness and would presumably slow the spread of transgenes into wild populations.