Salt-tolerant plants must be able to maintain low cytoplasmic Na$\sp+$ levels. In salt-tolerant Chara longifolia, enhanced Na$\sp+$ efflux plays an important role. When cultured in salt water, C. longifolia has a Na$\sp+$ efflux of 264 $\pm$ 14 nmol m$\sp{-2}$ s$\sp{-1}$ at pH 7, 13 times higher than the rate of freshwater-adapted cultures and 31 times higher than the rate of freshwater-obligate Chara corallina. As in freshwaeer-adapted plants, efflux is highest at pH 5, but pH dependence is less steep and more linear in cells adapted to salt water. The response to inhibitors is also different. In plants of both species from freshwater cultures, Na$\sp+$ efflux is inhibited by Li$\sp+$ at pH 5 but not 7 or 9, whereas in the salt-adapted C. longifolia, Li$\sp+$ inhibits at pH 7 and 9 but not at pH 5. Amiloride inhibits efflux in the salt-adapted cells but not in cells from freshwater culture. We therefore conclude that a new type of Na$\sp+$ efflux system is induced under salt stress, although both systems have characteristics suggestive of a Na$\sp+$/H$\sp+$ antiport. Efflux of Na$\sp+$ from C. longifolia may be due to the activity of a Na$\sp+$/H$\sp+$ antiport driven by the primary active transport of the P-type H$\sp+$-ATPase, and/or by the activity of a Na$\sp+$-ATPase. We attempted to isolate P-type ATPases from C. longifolia using a PCR-based cloning strategy, and obtained two clones. The first had homology to prokaryotic cation-transporting ATPases and eukaryotic Na$\sp+$/K$\sp+$ ATPases. The second showed high homology to the B subunit of the E. coli high affinity K$\sp+$-ATPase kdp. Taken together, these studies suggest that molecular cloning and expression analysis using Chara is feasible and may reveal an interesting suite of ion transport mechanisms at the molecular genetic level.