The demonstrations by Losey et al. (1)and Hansen and Obrycki (www.ent.iastate.eduyentsocynch 99yprogyabsy D81.html) that milkweed leaves dusted with heavy concentrations of Bt corn pollen are toxic to Monarch butterf ly larvae (Danaus plexippus) feeding on them were consistent with the known toxicity of Bt endotoxin to Lepidoptera in general and the expression of Bt endotoxins in the pollen of the strains of corn they studied. Much speculation and some investigations followed, concerning the extent to which the poisoning of Monarch butterf lies and other nontarget Lepidoptera might be sig-nificant contributors to the mortality of these insects in nature. For example, Shel-ton and Roush (2) were critical of the two earlier findings, but did not provide any data from nature, despite the fact that Losey et al. (1), in the original report, stated ''it would be inappropriate to draw any conclusions about the risk to Monarch populations in the field based solely on these initial results.''In a recent issue of PNAS, Wraight et al. (3) reported their experiments with pop-ulations of black swallowtail larvae (Pap-ilio polyxenes) under field conditions. The food plants were located at varying dis-tances from plantings of Bt corn; the authors found no effects on the mortality of the larvae. These results demonstrated that Bt corn pollen from this corn strain is not toxic to this species of butterf ly at levels observed in the field no matter how close the larval food plants were to the pollen-shedding corn plants. It has not been demonstrated, however, that Mon-arch butterf lies and black swallowtails are equally susceptible to Bt endotoxins. Further, the pollen of the corn strain 176 used by Hansen and Obrycki (www.ent. iastate.eduyentsocynch 99yprogyabsy D81.html) in their experiments with Mon-arch butterf lies was demonstrated by Wraight et al. (3) to be lethal to black swallowtail larvae in the laboratory, whereas the pollen of the strain they used in their field experiments, with 1y40 of the Bt endotoxin level of strain 176, was not. Studies of the effects of corn strain 176 pollen on both black swallowtails and Monarch butterf lies in the field are un-derway in the summer of 2000. For their original report, Losey et al. (1) used still a different corn strain, N4640. At any event, the level of Bt endotoxin in the pollen of the particular corn strain, as expected, has a direct effect on the survival of black swallowtail larvae in the laboratory and presumably on the larvae of other butter-fly species as well. It has not been demonstrated whether different species of butterflies have varying levels of tolerance to Bt toxin, but they probably do, as demon-strated with other toxins.