The aim of the study reported here was to evaluate the performance and variability of transgenic potato clones. For the genetic transformation the Polish potato cultivar Irga was chosen in order to improve resistance to a necrotic strain of potato virus Y (PVYN ) by introducing to its genome a truncated gene coding PVYN replicase (in sense and antisense orientations). Transgenic plants and clones derived from them were propagated and the obtained tubers were planted in a replicated field trial. The several agronomic and morphological traits were evaluated and compared with measurements for non-transgenic control plants. The traits of transgenic clones showed a much greater variability than non-transgenic plants. The variability depended on the type of the introduced construct (in this case it was the orientation of the construct). None of the transgenic clones turned out to be completely true to type and resistant to PVYN, but some resistant clones expressed deviations in a small proportion of the traits. Usually, deviations were observed for those traits whose inheritance is characterised by a large environmental component. Genetic transformation is an effective method for introducing resistance. However, the method causes a great variability, which makes selection among transgenic clones a necessary step in breeding of an improved transgenic cultivar. Such selection has many similarities with selection done among traditionally obtained clones.