In the present study, transformation of potato tuber cells by Agrobacterium tumefaciens and Ti plasmid treatment was studied. The results are as follows: Tumors were induced in potato tuber discs by infection with Agrobacterium tumefaciens KU-12. Whole plants were regenerated from the tumors on hormone-free medium. It was identified that the lysopine dehydrogenase (LpDH) activity was present in the tumor-derived plants. A total of 165 calli were obtained from Ti-plasmid DNA(pTi-12)-treated potato protoplasts on hormone-free medium and a few of them formed a root. Each of the non-rooted and rooted callus was randomly selected and tested for the presence of LpDH activity. Both of them were ocotopine positive, indicating that they were transformants. In order to analyze the T-DNA region maintained in transformed plant genome, the DNA-DNA hybridization experiment was performed. As a result, it was found that Sma I fragments of Ti plasmid (pTi-12) 13.4 kb, 3.7 kb, and 4.75 kb in size, were completely contained within T-region, and each fragment of 9.7kb and 4.55 kb was joined to opposite ends of T-region. Also, it was identified that Sma I fragments which were contained within T-DNA region were covalently linked together within the pTi-12 DNA molecule.