A full-length cDNA homologue of SOS1 (salt overly sensitive 1) was isolated from the salinity-tolerant species Chrysanthemum crassum and found to encode a Na+/H+ antiporter, using degenerate PCR and RACE-PCR. The 3752-bp sequence comprised a 3438 bp open reading frame, encoding a 127-kDa protein with 12 transmembrane domains within its N terminal portion, and a hydrophilic cytoplasmic tail in its C-terminal portion. CcSOS1 appears to be a plasma membrane protein, and shares ∼62% identity at the peptide level with its Arabidopsis thaliana homologue. Expression of CcSOS1 in the roots of C. crassum was sensitive to salinity stress, while in the leaves CcSOS1 was down-regulated in the presence of abscisic acid. CcSOS1 transcript abundance was reduced in both roots and leaves of plants exposed to low temperature, while it was increased in leaves (but not in roots) after drought stress. CcSOS1 expression was not regulated in the presence of CaCl2. A heterologous complementation assay in yeast suggested that CcSOS1 directs Na+ efflux, mimicking the function of the endogenous NHA1 protein. Thus CcSOS1 appears to encode a salinity-inducible plasma membrane Na+/H+ antiporter. This gene may be useful in transgenic approaches to improving the salinity tolerance of related ornamental species.