The colchicine-induced hexaploid (2n=72) from the male-sterile triploid hybrid Solanum polytrichon x S. phureja is also male sterile with the tetrad type of male sterility as is the case with its triploid counterpart. The chromosome pairing in hexaploids is almost regular with an average of 0.3 IV, 0.2 III, 32.2 II and 5.8 I per cell. Thus tetrad male sterility of the hybrids proved to be controlled by plasmon-genic interaction. S. polytrichon has ‘sensitive’ [Trs] plasmon whereas S. phureja possesses the ‘resistant’ [trr] plasmon. Plasmon-genic sterility does not seem to occur infrequently in hybrids with the Mexican Solanum species as plasmon donors. Such Mexican species seem to be isolated from the South American ones by spatial, genomic and plasmon barriers. If the genes for resistance to Phytophthora are to be transferred from the Mexican to South American species, it is advisable to use Mexican species as male partners in the interspecific hybridization in order to avoid the involvement of their plasmons.