Technical Abstract: Limiting infestations of cereal aphids is a key to preventing yield loss in wheat, Triticum aestivum L. Host-plant resistance, derived from conventional breeding methods or potentially through application of biotechnology, is one strategy for managing cereal aphids in wheat. The objective of our study was to evaluate population growth of the bird cherry-oat aphid, Rhopalosiphum padi L., on conventional, non-transgenic wheat varieties and on transgenic wheat lines capable of expressing potato proteinase inhibitor II. In a set of assays with conventional varieties, `Sharp,' `Marshall' and `Ember' were relatively resistant to population growth of R. padi compared to `2375' and `Russ.' A second set of assays with conventional varieties showed that Sharp and Marshall limited R. padi-population growth compared to the varieties `Butte 86,' `Guard,' `Ivan' and `Prospect.' In a third set of assays that included transgenic isolines of Prospect wheat, population growth of R. padi on isolines subjected to the transformation procedure was equal to or greater than that on non-transformed Prospect wheat. In two of those assays, low numbers of alatoid R. padi were found on isolines that had been subjected to the transformation procedure but not on non-transgenic Prospect wheat. In summary, three contemporary wheat cultivars are currently available with resistance to R. padi, but a better understanding of expression of proteinase inhibitors in transgenic wheat is needed before these lines can be of use against R. padi.