1.  The population abundance and age structure of Rhopalosiphum padi, one of the most common maize aphid species, on transgenic Bt (expressing the Cry1Ab protein) and non-Bt isogenic maize was studied in commercial plots during three crop seasons.2.  A higher density of aphids, particularly alates and young nymphs, occurred in Bt plots at very young maize development stages, corresponding to the settlement period, in the 3 years studied. Possible causes are discussed. After this period, there were no differences between Bt and non-Bt maize.3.  Mortality, development, and reproduction of the offspring of alate forms of R. padi and the offspring of different generations of apterous forms fed with Bt maize were evaluated in the laboratory under controlled conditions.4.  The developmental and pre-reproductive times of the offspring of the first generation of alatae were shorter and the intrinsic rate of natural increase (rm) higher when aphids fed on Bt maize. The opposite occurs with the offspring of the first generation of apterous mothers, which have lower nymphal and adult mortality, shorter developmental and pre-reproductive times, a higher effective fecundity rate, and greater rm, when fed on non-Bt maize. The differences in aphid development on the two cultivars may be linked to changes in host-plant quality due to pleiotropic effects of the genetic modification.5.  No differences on aphid mortality, developmental and pre-reproductive times, fecundity, and rm were found between the offspring of apterous aphids maintained on Bt or non-Bt maize for several generations.