The Arabidopsis phyB, phyD, and phyE phytochromes regulate plant developmental and growth responses to continuous red light (R) and to the ratio of R to far-red (FR) light. The activities of these three photoreceptors in the control of seedling growth have been compared using a transgenic assay based upon induction of R-hypersensitivity of hypocotyl elongation by overexpression of the apoproteins from the 35S promoter. 35S-phyB, 35S-phyD, and 35S-phyE lines expressing similar levels of the respective phytochromes were isolated. Under pulses of R, phyB is very active in inducing a dwarf hypocotyl phenotype, whereas phyD and phyE are inactive. Under high-fluence continuous R, phyD shows a gain in activity whereas phyE does not. These results demonstrate significant differences in the inherent regulatory activities of these receptor apoproteins. To localize the sequence determinants of these functional differences, chimeric proteins were constructed by shuffling amino-terminal, central, and carboxy-terminal regions of phyB and phyD. Overexpression analysis of the phyB/D chimeras shows that it is the central region of these proteins that is most critical in determining their respective activities.