Protoplasts from diploid S. tuberosum and diploid S. phureja were electrofused followed by selection of the heterokaryons with a micromanipulator. Visual identification of the heterokaryons was facilitated by fluorescein diacetate staining of the protoplasts from one of the parents, which was grown on herbicide containing medium to induce bleaching of the chlorophyll. In total, 840 heterokaryons showing red (chlorophyll) and yellow-green (fluorescein) fluorescence were selected and cultured at relatively low densities using various feeder systems. Finally, 18 putative hybrid plantlets were obtained and grown to maturity. DNA histograms indicated that the plants were hexaploid, octoploid or mixoploid. With Giemsa C-band pattern analysis of the chromosomes the hybrid character and the combinations of the chromosome sets of all plants investigated could be established.