Abstract: Transgenic corn expressing Bacillus thuringiensis (Bt) toxin Cry1Ab has been plantedin Spain to control corn borers as Ostrinia nubilalis (Hübner), since 1998. Indeed, 79 thousandhectares have been planted to Bt-maize in different Spanish areas in 2008. The high selectivepressure may produce the development of resistance in populations of the target pest, decreasingthe effectiveness of the transgenic crop. The selection may be even higher in neighbour crops inwhich O. nubilalis is a pest and where it is controlled by conventional Bt-spray productscontaining Cry1Ab.A sample of O. nubilalis was collected from a greenhouse in south-eastern Spain that wassuffering high levels of infestation after repeated treatments with different Bt products. Insectswere brought and reared in our laboratory and their susceptibility to activated Cry1Ab toxin,Cry1Ab protoxin and the Bt standard HD-1-S-2005 product was tested. As a susceptible control,insects from France kept in the laboratory for more than 10 years without exposure to Bt wereused. The “effective growth inhibition” was recorded seven days after treatment and accountedfor both dead larvae and larvae which not passed the first instar. PROBIT analyses of the datarevealed no significant different response between the strains to activated Cry1Ab toxin and toHD-1-S-2005. Cry1Ab protoxin showed 7-fold lower activity in the laboratory strain whencompared to the field strain. These data suggest an absence of a relevant shift in the resistance toCry1Ab in the insects from the field strain as compared to the laboratory one, and point to adeficient Bt product application in the greenhouses.