Salt-tolerant cell lines of citrus rootstock (Poncirus trifoliata cv Pomeroy) were selected by subculturing embryo-derived calli on media containing sublethal concentrations of NaCl (5 and 10 g/l). Selected lines showed a normal growth in the presence of salt at the concentrations used for selection, and salt tolerance persisted after a passage on a salt-free media. Their K(+) and Ca(2+) content remained higher than in control cells for increasing NaCl concentration in the medium, suggesting a modification of cell membrane permeability as the main cause of NaCl tolerance. Shoots and plants regenerated from selected cell lines showed improved growth and salt tolerance. Calli induced from these plants tolerated a salt concentration of 10 g/l, indicating the persistance of the selected trait.