Glycinebetaine (betaine) highly accumulates as a compatible solute in certain plants and has been considered to play a role in the protection from salt stress. The betaine biosynthesis pathway of betaine-accumulating plants involves choline monooxygenase (CMO) as the key enzyme and phosphoethanolamine N-methyltransferase (PEAMT), which require S-adenosyl-L-methionine (SAM) as a methyl donor. SAM is synthesized by SAM synthetase (SAMS), and is needed not only for betaine synthesis but also for the synthesis of other compounds, especially lignin. We cloned CMO, PEAMT and SAMS isogenes from a halophyte Atriplex nummularia L. (Chenopodiaceous). The transcript and protein levels of CMO were much higher in leaves and stems than in roots, suggesting that betaine is synthesized mainly in the shoot. The regulation patterns of transcripts for SAMS and PEAMT highly resembled that of CMO in the leaves during and after relief from salt stress, and on a diurnal rhythm. In the leaves, the betaine content was increased but the lignin content was not changed by salt stress. These results suggest that the transcript levels of SAMS are co-regulated with those of PEAMT and CMO to supply SAM for betaine synthesis in the leaves.