We present the first evidence that transgenic Bacillus thuringiensis (Bt) corn pollen naturally deposited on Asclepias syriaca, common milkweed, in a corn field causes significant mortality of Danaus plexippus L. (Lepidoptera: Danaidae) larvae.  Larvae feeding for 48 hours on A. syriaca plants naturally dusted with pollen from Bt corn plants suffered significantly higher rates of mortality at 48 hours compared to larvae feeding on leaves with no pollen, or feeding on leaves with non-Bt pollen.  Mortality at 120 hours of D. plexippus larvae exposed to 135 pollen grains/cm2 of transgenic pollen for 48 hours ranged from 60-70%.  We found no sub-lethal effects on D. plexippus adults reared from larvae that survived a 48 hour exposure to three concentrations of Bt pollen.  Based on our quantification of the wind dispersal of this pollen beyond the edges of agricultural fields, we predict that the effects of transgenic pollen on D. plexippus may be observed at least 10 m from transgenic field borders.  However, the highest larval mortality will likely occur on A. syriaca plants in corn fields or within 3m of the edge of a transgenic corn field.  We conclude that the ecological effects of transgenic insecticidal crops need to be evaluated more fully before they are planted over extensive areas.