Cultured tobacco (Nicotiana tabacum L. cv. Wisconsin 38) cells were found to have altered DNA contents and chromosome numbers after adaptation to NaCl. Cells adapted to 428 mM NaCl were predominately hexaploid compared to the normal tetraploid 2N(2C)=4X=48 chromosome number of unadapted cells. Enrichment of the cell population for hexaploid cells occurred only after exposure to higher NaCl (428 mM), not lower levels of NaCl (171 mM). The majority of adapted cells remain hexaploid for at least 25 cell generations after removal from NaCl exposure. Adapted cell populations were found to have fewer cells with highly polyploid (2N≥96) nuclei. Salt tolerance of hexaploid cells was not found to be significantly greater than that of tetraploid cells. Cells with higher ploidy levels were less salt tolerant. It is suggested that high levels of NaCl induce polyploidization and that exposure to NaCl selects against cells with very high ploidy levels.