A PCR approach was used to isolate a phytoene synthase (Psy) cDNA from sunflower (Helianthus annuus L.). A reconstructed full-length sequence (1598 bp) of the Psy cDNA was obtained; it contained a 1242 bp CDS, 172-nucleotides of 5'-untranslated region (UTR), and 170-nucleotides of 3'-UTR. The predicted protein (46.8kDa) displayed a sequence of 414 amino acid residues with a putative transit sequence for plastid targeting in the N-terminal region. Phylogenetic analysis demonstrated that the sunflower Psy (HaPsy) clustered with the marigold (Tagetes erecta) Psy gene, with which it showed an overall amino acid identity of 93.7%. Moreover, the HaPsy sequence relates closely with other Psy sequences of higher plants. HaPsy was highly expressed in cotyledons, and young and mature leaves. In contrast, HaPsy transcript levels were comparatively lower in the stem and almost absent in the roots. The HaPsy transcript levels were influenced by leaf expansion, which suggested that the expression of the HaPsy gene is regulated during the process of leaf development. The role of HaPsy in controlling carotenoid biosynthesis is demonstrated by the concurrent increase of HaPsy transcript levels with the light-dependent enhanced carotenoid production in green tissues of sunflower.