The circadian clock controls several important processes in plant development, including the phase transition from vegetative growth to flowering. In Arabidopsis thaliana, the circadian-regulated gene CONSTANS (CO) plays a central role in the photoperiodic control of the floral transition, one of the most conserved flowering responses among distantly related plants. CO is a member of a plant-specific family of transcription factors, and when it arose during the evolution of higher plants is unclear.A CO homologous gene present in the genome of the unicellular green alga Chlamydomonas reinhardtii (CrCO) can complement the Arabidopsis co mutation and promote early flowering in wild-type plants when expressed under different promoters. Transcript levels of FLOWERING LOCUS T (FT), the main target of CO, are increased in CrCO transgenic plants in a way similar to those in plants overexpressing CO. In the microalga, expression of CrCO is influenced by day length and the circadian clock, being higher in short photoperiods. Reduction of CrCO expression in Chlamydomonas by RNA interference induces defects in culture growth, whereas algae induced to express high levels of CrCO show alterations in several circadian output processes, such as starch accumulation and the onset of expression of genes that regulate the cell cycle.The effects observed may reflect a conserved role for CrCO in the coordination of processes regulated by photoperiod and the circadian clock. Our data indicate that CO orthologs probably represent ancient regulators of photoperiod-dependent events and that these regulators arose early in the evolutionary lineage that gave rise to flowering plants.