In many species of crop plant, gene flow by cross-pollination is possible between spatially separate fields. To preserve a crop's varietal purity or to restrict ingress into conventional varieties of genetically modified (GM) genes, a quantitative understanding of gene flow is useful. Previous measurements of gene flow in safflower (Carthamus tinctorius L.), a crop with GM varieties, were made in plots of less than 1 ha. Here, I evaluate a mathematical model of field-to-field gene flow due to insect pollination using parameter values appropriate to a large agricultural field of safflower. The model was solved based on laboratory pollination experiments and observations made on a large (40 ha) safflower field in Lethbridge, Canada that was pollinated by honey bees (Apis mellifera) and bumble bees (Bombus spp.). The model estimated the maximum feasible level of bee-mediated, field-to-field gene flow to range between 0.05% and 0.005% of seed set (95% upper confidence intervals of 0.23% and 0.023%), depending on the composition of the bee fauna. These relatively low values emerged for two reasons: safflower has a high capacity for automatic self-fertilization; and bees undertook long foraging bouts in the field, which made between-field pollinations relatively rare. A strategy for minimizing GM gene flow should utilize a conventional safflower variety that has a high capacity for automatic self-fertilization and should allow the conventional plants to grow in large stands to encourage long foraging bouts by bees.