A family of CONSTANS LIKE genes (COLs) has recently been identified in Arabidopsis thaliana and other plant species. CONSTANS, the first isolated member, is a putative zinc finger transcription factor that promotes the induction of flowering in A. thaliana in long photoperiods. Phylogenetic analysis of the COL family demonstrated that it is organized into a few distinct groups, some of which evolved before the divergence of gymnosperms and angiosperms. Molecular evolutionary analyses showed that COL genes within the Brassicaceae family evolve rapidly. The number of nonsynonymous substitutions was larger, and the ratio of nonsynonymous to synonymous substitutions was higher. The analysis also indicated that the rate of evolution is heterogeneous between different domains in the COL genes. The results support previous data indicating that plant regulatory genes evolve relatively fast and that the rate of evolution varies significantly between different regions of those genes. The rate of evolution of COL genes seems to have accelerated during later stages of evolution, possibly as an effect of frequent gene duplications.