Plants synthesize the osmoprotectant glycine betaine (GB) via choline betaine aldehyde glycine betaine [1]. Two enzymes are involved in the pathway, choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH). A full length CMO cDNA (1,643bp) was cloned from Amaranthus tricolor. The open reading frame encoded a 442-amino acid polypeptide, which showed 69% identity with CMOs in Spinacia oleracea L. and Beta vulgaris L. DNA gel blot analysis indicated the presence of one copy of CMO gene in the A. tricolor genome. The expressions of CMO and BADH proteins in A.tricolor leaves significantly increased under salinization, drought and heat stress (42 o C), as determined by immunoblot analysis, but did not respond to cold stress (4 o C), or exogenous ABA application. The increase of GB content in leaves was parallel to CMO and BADH contents.