We have isolated a new complementation group of Arabidopsis thaliana long hypocotyl mutant (hy6) and have characterized a variety of light-regulated phenomena in hy6 and other previously isolated A. thaliana hy mutants. Among six complementation groups that define the HY phenotype in A. thaliana, three (hy1, hy2, and hy6) had significantly lowered levels of photoreversibly detectable phytochrome, although near wild-type levels of the phytochrome apoprotein were present in all three mutants. When photoregulation of chlorophyll a/b binding protein (cab) gene expression was examined, results obtained depended dramatically on the light regime employed. Using the red/far-red photoreversibility assay on etiolated plants, the accumulation of cab mRNAs was considerably less in the phytochrome-deficient mutants than in wild-type A. thaliana seedlings. When grown in high-fluence rate white light, however, the mutants accumulated wild-type levels of cab mRNAs and other mRNAs thought to be regulated by phytochrome. An examination of the light-grown phenotypes of the phytochrome-deficient mutants, using biochemical, molecular, and morphological techniques, revealed that the mutants displayed incomplete chloroplast and leaf development under conditions where wild-type chloroplasts developed normally. Thus, although phytochrome may play a role in gene expression in etiolated plants, a primary role for phytochrome in green plants is likely to be in modulating the amount of chloroplast development, rather than triggering the initiation of events (e.g., gene expression) associated with chloroplast development.