The CONSTANS (CO) gene of Arabidopsis plays a key role in the photoperiodic flowering pathway. To investigate photoperiod responses in cereals in more detail, we isolated three kinds of CO/Hd1 (rice ortholog of CO) homolog from hexaploid wheat, derived from the A, B, and D genomes and designated as wheat ortholog of CO from A genome (TaHd1-1), TaHd1-2, and TaHd1-3, respectively. They were highly similar to each other and to Hd1, and in addition harbored two conserved regions: two zinc finger motifs and CONSTANS, CONSTANS-LIKE and TIMING OF CAB EXPRESSION 1 (CCT) domain like CO/Hd1. They were located on the long arm of the homoeologous chromosome 6. TaHd1-2 harbored a 63-bp deletion at the promoter region containing the GATA-1 box, and consequently, we detected no subsequent transcript. The TaHd1-1 genomic clone was introduced to a rice line deficient in Hd1 function. Transgenic plants complemented the functions of rice Hd1: they promoted heading under short-day (SD) conditions and delayed it under long-day (LD)/natural conditions, indicating that Hd1 proteins from SD and LD plants share common structures and functions.