Abstract: Day length controls development in many plants. In Arabidopsis thaliana, the CONSTANS (CO) gene is a key component in the photoperiodic pathway controlling floral transition. A novel cDNA encoding a CO homolog was isolated from Nicotiana tabacum cv. Kutsaga Mammoth 10 and designated NtCO1 (GenBank accession number, JN022535.1). The full cDNA sequence was 1493 bp with an open reading frame (ORF) of 1211 bp, encoding a protein of 403 amino acids. The predicted NtCO1 protein contained two B-box-type zinc fingers and a CCT domain. Analysis based on amino acid sequence alignment showed that NtCO1 shared high identity with StCO (86.5%) from Solanum tuberosum, AtCO (50%) from A.thaliana and Hd1 (43.7%) from Oryza sativa. Semi-quantitative RT-PCR analysis showed that NtCO1 was expressed specifically and strongly in leaf tissues but weakly in stem and root tissues.