Mathematical models estimated that xenogamy accounted for 7% and 35% of the stigmatic pollen loads ofIpomoea hederaceaandI. purpurea, respectively, in experimental populations. The xenogamy estimate forI. hederaceaagreed closely with outcrossing estimates previously reported for this species. The discrepancy between the xenogamy estimate and the previously reported outcrossing estimate forI. purpureacould be explained by differing pollinator flight patterns between experimental and natural populations and/or by selection for cross pollen in the pistil ofI. purpurea.Interspecific pollen flow fromI. purpureato emasculated flowers ofI. hederaceareduced seed production in the latter. The possible significance of interspecific pollen flow fromI. purpureafor the evolution of autogamy inI. hederaceawas discussed.