Insecticidal toxins from Bacillus thuringiensis (Bt) can now be deployed either in sprays or transgenic plants. Some entomologists and environmentalists have argued that the sprays are preferable to plants because they are less likely to cause resistance. However, Bt sprays are not generally competitive with chemical insecticides and seem unlike1.v to displace them. In contrast, transgenic plants appear to be sufficiently effective to displace chemicals, making such plants attractive from the standpoint of environmental protection. Further, simulation models using data from the diamondback moth and a laboratory experiment using the Indianmeal moth suggest that under at least some circumstances. transgenic plants bearing only one Bt gene may be more effective than sprays for delaying resistance to Bt Resistance in a laboratory-selected strain of the Colorado potato beetle is especially interesting because a strain that can survive Bt sprays and develop to maturity cannot develop successfully on transgenic plants, not even on some showing very low expression. This suggests that more mechanisms are available for resistance to sprays than to transgenic plants, perhaps making resistance to sprays more 'evolutionarily available'. When the use of two toxin genes is considered, simulation models suggest that transgenic plants may be much more durable than sprays of similar efficacy.