The performance and flight behavior of the potato aphid Macrosiphum euphorbiae was studied on the 'Superior-BT' line transgenic for the CryIIIA toxin of Bacillus thuringiensis (BT), resistant to the Colorado potato beetle; and non transformed 'Superior' line which served as control.  Mortality of the treated aphids was negligible and potato lines did not affect the development time of M. euphorbiae, but aphids were largest on 'Superior' and smallest on BT potatoes.  This difference was reflected in aphid fecundity, which was lowest on 'Superior-BT', and highest on Superior.  Incidence of flight in newly emerged late M. euphorbiae that developed on BT was high compared to control.  The results illustrate that the performance of a secondary pest of potato can be unpredictably affected by the resistance factor involved in developing specific resistance to a primary pest.