Phytochrome, a red/far-red-light photoreceptor protein of plants, is encoded by a small gene family. Phytochrome A (PHYA), the product of the PHYA gene, is the predominant molecular species of phytochrome in etiolated tissue and has been best characterized biochemically. To define a role for PHYA, we isolated new mutants, designated fre1 (far-red elongated), in Arabidopsis thaliana that were specifically deficient in PHYA spectral activity and protein accumulation. These mutants were identified on the basis of their long hypocotyl phenotype under continuous far-red light. Although the fre1 mutants lacked the hypocotyl response to continuous far-red light, their responses to continuous white light and to end-of-day far-red-light treatments were normal. Thus, PHYA appears to play only a minor role in the regulation of hypocotyl elongation under natural conditions. In contrast, the fre1 mutation affected greening a fre1 mutant was less able than the wild type to deetiolate after growth in the dark. However, the potentiation effect of a red-light pulse on accumulation of chlorophyll was not changed significantly in the fre1 mutants. Thus, the function of PHYA might be highly specialized and restricted to certain phases of Arabidopsis development.