ABSTRACT: Fruits of transgenic cucumber lines expressing preprothaumatin II gene were evaluated concerning their aroma. Four homozygous lines, that is, 210 06, 212 01, 224 09, and 225 03 with different levels of transgene expression were selected. Recipient line cv. Borszczagowski, which was formed by inbred line of Cucumis sativus L., was used as a control. The experiment was carried out in a greenhouse and an outdoor experimental plot. The aroma of cucumber fruits was evaluated by GC/MS, as well as GC/MS/TOF in the distillates and by SPME. Irrespective of the isolation/separation technique used, the differences between aroma compounds in transgenic cucumbers and the control were quantitative, and not qualitative. Modified samples showed higher concentrations of volatiles, particularly of the main cucumber fruits odorant (E, Z)-2,6 nonadienal. Transgenic expression of the thaumatin II gene resulted not only in a sweeter taste of fruits in comparison with the control, but also higher aroma acceptability. This was shown by sensory profile analysis. Also electronic nose measurements differentiated between transgenic lines and the control.