Phytochrome A (PhyA) mediates most, if not all, various plant responses to far-red (FR) light. Here, we report a novel genetic mutation that impairs a variety of responses in the PhyA-signaling pathway of Arabidopsis thaliana. The mutation was isolated by screening seedlings that show reduced sensitivity to continuous far-red (FRc) light irradiation, but not to continuous red (Rc) light irradiation. The mutation named fin2-1 is not allelic to a PHYA mutation. Furthermore, immunoblot analysis indicated that the amount of the phytochrome A apoprotein in the fin2-1 mutant was comparable to that in wild type. Seedling of the fin2-1 mutant showed defects in hypocotyl growth inhibition and apical hook and cotyledon opening in FRc light but not in Rc light. The results showed that the mutation occurred in a downstream signaling component potentially specific to PhyA. Other PhyA-mediated responses such as FR-preconditioned blocking of greening, anthocyanin accumulation, reduction of gravitropic response, and expression of the CAB and CHS genes were impaired by the fin2-1 mutation: the degree of the mutant effect on the responses was variable. However, FR light-mediated seed germination and photoperiodic flowering responses were not affected significantly in the mutant. These results showed that FIN2 defines an upstream branch point in the PhyA signaling pathway.